The mechanical room is two floors below street level, behind a fire door, and has no bars. You have finished the repair. The building manager is standing there with his keys out, ready to sign off and go home, and he says the thing that decides whether you get paid this month: "Can you leave me the invoice?"

At a desk, that is a two-minute task. In a basement, on a phone, with wet gloves and 12% battery, it is a different problem — the part of invoicing desktop software never solved, and the reason so many field service businesses bill a week late. Not from laziness. From physics.

This guide covers three things that make field invoicing work: billing on site, what "offline" really means, and how to handle clients who pay in a currency you do not live in.

Bill before you leave, or bill from memory

The single highest-return habit in field service is closing the invoice at the property.…