Portable Printers in Airline In-flight Retail Solution

How can a passenger print the receipt by themselves easily?
We make this happen in flight:
Flight attendants use this RG-MTP58B printer to provide printed receipts for customers who consume onboard and request paper receipts. As part of the total solution, MTP58B can connect to any mobile device (smart phones and tablets) to make it easy for airlines to do retail business in fight.