[user5454]

I recently returned from a trip to the USA and was a bit surprised that they asked for ZIP code instead of PIN for my credit card. While the NY subway accepted my non-US ZIP code, some gas stations did not as they apparently checked the entered ZIP code against valid US ZIP codes.

[Kurtz79]

To my knowledge, in some machines non-US ZIP codes are accepted up to a cash limit (can't remember if it's daily or for transaction).

Probably your bill at the gas stations was higher than the one for the subway tickets.