As an admin, navigate to Settings >Invoices >Payment Types:
Tap the blue plus button on the top right to add a new method.
Note: this could be Square, Clover, etc.
When this is done and you tap the Save button, this will be available as an option for your technicians to choose.
In the Pointman app:
The below is what technicians will see when they select the third party options:
They are able to select the approval date and amount. If there is a reference number from the 3rd party app, they can enter it here as well. They can also add a customer signature showing the customer has paid their bill.