In both HVAC and plumbing, you generally want to get paid at the end of the call. If you walk into a home with a clogged drain or no A/C and fix the problem, there’s no reason you shouldn’t fill out your invoice and get paid on the spot.
Immediate payment is par for the course when dealing with a new customer. An existing customer with whom you have a good relationship may prefer to have an invoice sent later, and it is fine practice to honor that if you know they’re good for it.
Some calls are trickier than others. If the call requires time to pass to make sure your solution was the effective, it’s not a bad idea to wait to send the invoice. You can explain to the customer that you want to wait it out to make sure the problem doesn’t happen again and they will be billed in the coming days—building trust this way will ensure the customer’s payment when the invoice arrives.