I don't think the zip code is actually used for any specific purpose - I ran into this problem at a gas station in Buffalo a while back and while I stood there thinking "I don't have a zip code", I suddenly realized I had one zip code in my head that I could remember - 90210 - who doesn't know that?! Anyway, I typed it in and went on pumping gas - it accepted it and had nothing to do with being connected to my credit card. I was once in NYC buying subway passes and the same thing happened when I used my credit card - entered 90210 and away I went!