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)