A printer was already mentioned, but a card reader with receipt printer would probably sell
In some respects they are already out there, as a separate unit communicating with your phone via Bluetooth or WiFi. I don't really see what you would get by more tightly integrating it into the phone - it would simply add a hell of a lot of bulk for something that would probably be better integrated as a separate unit in any event. You probably could reduce a card reader/printer down to perhaps half an inch depth at the cost of a standard and proprietrary paper size (something like a Post-It note). Even a small paper roll wold at least double that depth making the combined phone seem very bulky.
In any case, would you really want to hand over your own phone to customers dozens of times a day if there is not good reason to?
Biting the hand that feeds IT
