When I do sit in the back, I order what I want and when I am done ordering I simply hold up my EXP card.

Fortunately it doesnt happen too often (sitting in back). But more than once the FA has tried to run the card through her credit card machine. I even had who loudly announced "This card was rejected! Do you have a credit card that works?" When I explained what card she was holding, she simply gave me a second bottle of Absolut, apologized and continued on. My seatmates and I were hysterical.