Glossary definition

What’s the difference between a purchase order and an invoice?

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If you handle business purchases or payments, you’ve likely come across Purchase Orders (POs) and invoices. While they may look similar, they serve distinct purposes. 

Confusing the two can lead to mismatched paperwork, duplicate payments, and budget issues. Understanding their roles can help you improve cash flow, prevent unnecessary spending, and keep your financial records accurate. 

This guide breaks down the key differences between a PO vs. an invoice, why they matter, and how to manage them effectively: 

What is a purchase order? 

A purchase order, also known as a PO, is a formal document a buyer issues to a vendor as the first step to request goods or services. It includes key details such as item descriptions, quantities, prices, and payment terms.  

Once approved by the seller, a PO becomes a legally binding agreement, ensuring both parties adhere to the agreed terms. 

POs help you: 

• Establish clear expectations before a transaction. 

• Track spending and prevent unauthorized purchases. 

• Improve financial planning with pre-approved documentation. 

How does a purchase order work? 

A purchase order follows a structured process: 

1. The buyer identifies a need: a business determines the need for goods or services, such as office supplies, raw materials, or software licenses. 

2. A purchase requisition is created: internal teams may submit a request for approval before a PO is issued. 

3. The purchase order is generated: the PO includes all necessary details (vendor, items, quantities, cost, delivery date). 

4. The vendor reviews and accepts the PO: once accepted, it becomes a contract, and the vendor begins fulfillment. 

5. Goods or services are delivered: the vendor ships or provides the service, referencing the PO. 

6. The vendor issues an invoice: the invoice includes the PO number to help the buyer match the transaction and process payment. 

7. Payment is processed: the finance team verifies the invoice against the PO and goods received before approving payment. 

What information does a purchase order contain? 

A standard purchase order should always include certain items. If you’re creating one, the following are essential: 

• PO number: a unique identifier assigned to the order for tracking. 

• Buyer and seller information: contact details of both parties. 

• Item descriptions: a detailed list of goods or services being purchased. 

• Quantities and unit prices: the agreed number of items and cost per unit. 

• Subtotal, taxes, and total amount: the cost breakdown, including applicable sales tax and discounts. 

• Payment terms: agreed payment conditions, such as Net 30 or Net 60. 

• Delivery details: the expected shipping date, delivery location, and method. 

• Special terms or instructions: any specific conditions or additional requirements. 

When do businesses use purchase orders? 

You would use purchase orders whenever you need to formalize a purchase and maintain control over spending. POs are particularly useful in these scenarios: 

1. When a purchase requisition is approved 

Before issuing a PO, internal teams may submit a purchase requisition, which is an internal request for goods or services. Once approved by finance or procurement teams, a PO is generated and sent to the vendor. 

2. After selecting a vendor from multiple bidders 

If a company requests quotes from multiple suppliers, it will issue a PO to the selected vendor once a decision is made. This ensures that agreed-upon pricing and terms are documented. 

3. For large or recurring orders under a blanket PO 

Some companies use blanket purchase orders when ordering from the same vendor over a period of time. This simplifies procurement by setting pre-approved pricing and terms for multiple purchases without needing a new PO for each order. 

4. To improve financial planning and record-keeping 

Since purchase orders outline spending before a payment is made, they help businesses control budgets, avoid duplicate purchases, and simplify auditing. 

What is a purchase order number? 

A PO number is a unique reference number assigned to each PO. It acts as an order tracking tool, linking the PO to related documents such as invoices, packing slips, and receipts. They’re important for: 

• Streamlined tracking: ensures each transaction is recorded and traceable. 

• Efficient invoice matching: helps finance teams match invoices to corresponding purchase orders for payment processing. 

• Stronger vendor relationships: allows businesses to reference past orders easily, reducing disputes or miscommunications. 

Example of a PO number in action 

A retail company orders 500 units of packaging materials from a supplier. The PO is labeled PO-2024-00891. When the supplier sends an invoice for the completed order, they reference PO-2024-00891, allowing the company’s finance team to match the invoice to the original order before processing payment. 

What is an invoice? 

An invoice is a document that a seller issues to a buyer (once the purchase order has been fulfilled, for example) to request payment for goods or services that have been delivered. It serves as a bill and a record of the transaction, specifying how much the buyer owes, when payment is due, and how it should be made. 

Businesses use invoices to track sales, manage Accounts Receivable (AR), and ensure timely payments. If an invoice remains unpaid, it is classified as open, and once payment is received, it becomes closed or paid. 

How does an invoice work? 

The invoicing process typically follows these steps: 

• The seller delivers goods or services: after fulfilling an order, the seller prepares an invoice. 

• An invoice is issued to the buyer: the seller sends an itemized bill detailing what was provided, the total amount due, and payment terms. 

• The buyer processes the invoice: the buyer’s Accounts Payable (AP) team reviews and verifies the invoice before approving payment. Many businesses use invoicing software to automate this process, ensuring accuracy and speeding up approvals. 

• Payment is made: once approved, the buyer submits payment using the agreed method (e.g. bank transfer, credit card, ACH). 

What information does an invoice contain? 

A standard invoice includes the following details: 

• Invoice number: a unique reference number for tracking. 

• PO number: if applicable, the purchase order number that corresponds to the invoice. 

• Date of issue: the date the invoice is created. 

• Order date: the date the purchase was originally placed. 

• Shipment or delivery date: the date the goods or services were provided. 

• Buyer and seller information: names, addresses, and contact details of both parties. 

• Shipping details: if physical goods are involved, the shipping address and costs. 

• Payment terms: specifies conditions like Net 30 (payment due within 30 days). 

• Due date: the final date by which payment must be made. 

• Payment methods: bank transfer, credit card, PayPal, ACH, etc. 

• Line items: a breakdown of the goods or services provided, including: 

– Product/service

-Quantity

-Unit price

-Discounts (if applicable)

-Extended line-item total

• Subtotal: the total before tax and shipping fees. 

• Taxes: sales tax rate and amount (if applicable). 

• Shipping fees: any additional shipping or handling charges. 

• Total amount due: the final amount the buyer must pay. 

• Payments received (if applicable): if partial payments have been made, the remaining balance is listed. 

Many businesses use invoice templates found in accounting software to streamline the invoicing process and ensure consistency. Read our guide on how to write an invoice for more details. 

What is a purchase invoice? 

A purchase invoice is an invoice a business receives from a supplier after purchasing goods or services.  

Some businesses or finance teams may simply refer to it as an invoice, while others distinguish it from sales invoices to track expenses separately.  

It records what the business owes and is essential for managing accounts payable, budgeting, and verifying supplier charges.  

Purchase invoices often reference POs to ensure the goods or services received match what was originally requested. 

What is a sales invoice? 

A sales invoice is an invoice a business sends to a customer to request payment after delivering goods or services.  

It’s sometimes just called an invoice, but finance teams use “sales invoice” to differentiate outgoing invoices from purchase invoices.  

It helps businesses track accounts receivable, set clear payment terms, and maintain accurate revenue records.  

Sales invoices provide legal proof of a transaction, reducing disputes and ensuring timely payment collection. 

When do businesses use invoices? 

Invoices ensure businesses get paid and maintain accurate records. They’re used in different situations depending on the nature of the transaction. 

1. When goods or services are delivered 

Businesses typically issue invoices once they’ve fulfilled an order. For wholesalers, this happens after shipping products, while service providers send invoices after completing their work. If the transaction is on credit terms, the invoice remains open until payment is made. 

2. For immediate payment transactions 

When a customer pays upfront—whether in a retail store, an online checkout, or a business-to-business transaction using a company credit card—the invoice is considered paid at the time of purchase. In this case, it serves more as a receipt rather than an accounts receivable record. 

3. For tracking accounts payable and receivable 

Companies use invoices to track money going in and out. A purchase invoice records what a business owes its suppliers, while a sales invoice tracks what customers owe. Finance teams rely on these records for cash flow forecasting, reconciliation, and tax reporting. 

4. For recurring or subscription payments 

Businesses offering monthly SaaS software plans, leased equipment, or ongoing professional services often issue recurring invoices. These invoices may be automated and scheduled for regular billing, reducing administrative effort.  

Some companies replace invoices with payment receipts if customers are enrolled in automatic billing. 

5. For staged or milestone-based payments 

Industries like construction, manufacturing, and consulting often use invoices for progress payments. Instead of waiting until a project is finished, businesses invoice at key stages—for example, after completing design work, reaching a production phase, or delivering part of a contract. This ensures steady cash flow and project accountability. 

6. For international trade and customs clearance 

A proforma invoice is a pre-sale document that outlines estimated costs and terms before shipping goods. It’s often required for customs clearance, import licensing, or securing letters of credit. Though not legally binding, it helps buyers and suppliers agree on the details before finalizing a deal. 

What’s the difference between a purchase order and an invoice? 

Businesses rely on both purchase orders and invoices to manage transactions, but they play very different roles. While both documents record details of a purchase, one is used to initiate a transaction, and the other is used to request payment at the end.  

The table below breaks down the key differences between a PO and invoice: 

Feature	Purchase order	Invoice
Who issues it?	The buyer issues a PO to request goods or services.	The seller issues an invoice to request payment.
When is it created?	Before the transaction, when the buyer places an order.	After the transaction, once the goods or services have been delivered.
Purpose	Acts as a formal request to purchase goods or services.	Serves as a bill for payment after fulfillment.
Legal standing	Becomes legally binding once accepted by the seller.	Acts as a financial record but does not authorize a purchase.
Key details included	Contains order details, such as product descriptions, quantities, agreed prices, and delivery terms.	Lists the amount due, including a breakdown of items, payment terms, taxes, and due date.
Impact on financial records	Used in budgeting and expense tracking but does not require immediate payment.	Logged as AP for buyers and AR for sellers.
Payment status	No payment is made when a PO is issued—it only initiates the transaction.	An invoice demands payment, either immediately or by a due date.
How it’s used in a business process	Helps control spending by ensuring purchases are approved before they happen.	Ensures vendors are paid accurately and on time, preventing disputes.
Example scenario	A company orders office furniture from a supplier and issues a PO confirming the order.	The supplier delivers the furniture and sends an invoice for $5,000, requesting payment within 30 days.

How are purchase orders and invoices similar? 

While purchase orders and invoices serve different purposes, they share key similarities that help businesses manage transactions efficiently: 

• Both document transaction details: each includes buyer and seller information, item descriptions, quantities, prices, and payment terms. 

• Both support financial record-keeping: businesses use POs to track planned expenses and invoices to record completed transactions for accounting and audits. 

• Both help with budgeting and cash flow: POs outline expected costs, while invoices track payments due, helping businesses manage spending and revenue. 

• Both are legally significant: a PO becomes binding once accepted by a seller, while an invoice formally requests payment and can be used in disputes. 

• Both improve business efficiency: they create a structured process from order placement to payment, reducing errors and miscommunication. 

How do purchase orders and invoices work together? 

Businesses often use a three-way matching process, where a PO, invoice, and receiving report are compared before approving payment. This ensures that: 

• The invoice matches what was originally ordered. 

• The goods or services were received as expected. 

• The correct payment is processed, reducing the risk of errors or fraud. 

By linking POs and invoices, companies create a structured purchasing and payment workflow that improves inventory management, cash flow planning, and supplier relationships. 

Why do you need both purchase orders and invoices? 

Using both purchase orders and invoices may seem like extra paperwork, but together they create a transparent and accountable financial process. Businesses rely on them to: 

• Standardize procurement and payments: POs ensure spending is approved before purchases happen, while invoices formalize payment requests and prevent unauthorized transactions. 

• Improve inventory and expense tracking: POs help businesses monitor what’s been ordered, while invoices confirm what has been received and paid for, improving stock control and expense management. 

• Prevent disputes and miscommunication: purchase orders set clear expectations, reducing errors and ensuring that invoices match what was originally agreed upon before payments are processed. 

• Strengthen financial control and cash flow planning: POs provide visibility into upcoming expenses, while invoices help track outgoing payments, making it easier to manage budgets and take advantage of early payment discounts. 

• Simplify auditing and compliance: a clear record of POs and invoices helps businesses stay compliant with tax regulations, reduces fraud risk, and ensures a reliable paper trail for audits and dispute resolution. 

Common mistakes businesses make (and how to avoid them) 

Even businesses with strong procurement and payment processes can run into costly mistakes when managing purchase orders and invoices. These errors can lead to overpayments, cash flow issues, compliance risks, and strained supplier relationships. Below are some of the most common mistakes and how you can avoid them. 

1. Skipping purchase orders for smaller purchases 

Mistake: many businesses assume POs are only necessary for large expenses, leading to untracked spending and budget overruns on smaller purchases. 

Solution: set a minimum threshold for mandatory POs—for example, all purchases over $500 must have a PO. This ensures financial control while reducing unnecessary admin work. 

2. Paying invoices before verifying them 

Mistake: rushing to pay invoices without checking them against purchase orders and delivery receipts can result in overpaying, duplicate payments, or even paying for undelivered goods. 

Solution: implement a three-way matching process—comparing the PO, invoice, and delivery receipt before approving payment. Automate this step with accounting software to reduce errors. 

3. Failing to track PO and invoice numbers 

Mistake: without a structured numbering system, businesses lose track of outstanding invoices, due dates, and vendor payments, increasing the risk of missed or incorrect payments. 

Solution: use a sequential PO and invoice numbering system and integrate it into your accounting software for easy tracking. 

4. Not enforcing clear payment terms 

Mistake: invoices with vague or inconsistent payment terms lead to delays, disputes, and cash flow problems. 

Solution: standardize invoice templates with clear due dates, penalties for late payments, and early payment discounts to encourage prompt payment and avoid confusion. 

5. Missing early payment discounts 

Mistake: some suppliers offer discounts for early payments, but businesses often overlook these opportunities due to poor invoice tracking. 

Solution: set up automated reminders for invoices with early payment incentives, helping your business reduce costs while maintaining strong vendor relationships. 

6. Ignoring discrepancies between POs and invoices 

Mistake: invoices with incorrect pricing, missing items, or unexpected fees often go unnoticed until after payment is made, making dispute resolution difficult. 

Solution: set up an approval workflow where all invoices are reviewed for discrepancies before payment is issued. Flag and resolve any inconsistencies before processing. 

7. Not integrating procurement and accounting systems 

Mistake: manual data entry between procurement and accounting platforms leads to errors, slow processing, and missed invoices. 

Solution: use integrated accounting software to automate PO and invoice matching, eliminating manual work and improving accuracy. 

8. Failing to keep purchase orders and invoices for audits 

Mistake: businesses that don’t properly store POs and invoices risk compliance issues during audits or tax filings. 

Solution: digitize and store all POs and invoices in an accounting system with searchable records, making retrieval easy during audits. 

9. Damaging vendor relationships with delayed payments 

Mistake: late payments frustrate suppliers, leading to stricter payment terms, higher prices, or even supply chain disruptions. 

Solution: establish a clear accounts payable process to ensure timely invoice approvals and payments. Consider automating reminders to avoid delays. 

How Sage can simplify purchase order and invoice management 

By now, you know how important purchase orders and invoices are for keeping your business running smoothly. But managing them manually? That’s where things get tricky. Lost paperwork, missed approvals, and payment delays can cause headaches—and cost you money. 

With our purchase order software, you can automate the entire process—from creating and approving purchase orders to matching invoices and tracking payments. Three-way matching ensures you only pay for what you’ve received, while real-time reporting gives you full visibility over your spending. No more chasing paperwork or second-guessing your numbers. 

POs vs. invoices FAQs 

Can a purchase order be used as an invoice? 

No, a purchase order cannot be used as an invoice. A PO is created before a purchase to authorize a transaction, while an invoice is issued after to request payment. 

What happens if an invoice doesn’t match the purchase order? 

If an invoice doesn’t match the PO, businesses should review the discrepancy before making payment. Using a three-way matching process—comparing the PO, invoice, and receiving report—helps flag issues like incorrect pricing or missing items. 

Do I need both a purchase order and an invoice? 

Not always, but in many cases, using both ensures financial accuracy and control. Some businesses may allow purchases without a PO for low-value or recurring expenses, but for larger transactions, having both helps prevent errors, disputes, and unauthorized spending. 

Is a PO an invoice? 

No, a PO and an invoice serve different purposes. A PO confirms an order, while an invoice requests payment after the order has been fulfilled. 

Can you invoice without a purchase order? 

Yes, but it depends on company policy. Some businesses require POs for every purchase, while others allow invoices without a PO for smaller or recurring expenses. 

Is a purchase order proof of payment? 

No, a purchase order is not proof of payment—it only confirms an order was placed. Payment is only recorded once an invoice is processed and paid.