To expand on a comment by brhans
Something which apparently works for Canadians (with alphanumeric
postal codes) paying for fuel in the USA (where the gas pump wants a
5-digit zip code), is to just enter the numbers from their postal
code, followed by 0's. So a Canadian postal code of "A2B 3C4" would be
entered as "23400".
I can personally attest that in 2014, while living in the UK at a place with postcode*
AB16 2YZ, while visiting the US I was able to get an automated gas pump to accept
16200 as the zipcode for my credit card.
Another thing I have heard anecdotally is that for non-US cards the zipcode isn't actually checked, so just use
90210 (or whatever favourite zipcode you know)
* (not actually those letters)