[rietta]

It's not true that zip codes are unique to a particular city and state in the United States. Where I live, for instance, several couple of cities share the same post office - and thus zip code. It does cause confusion in some instances, especially with address verification.

There have been times when I have had to put the wrong city in my address because of zip code enforcement. Fortunately, the post office does a pretty good job delivering anyway.

[npcomplete]

Good to know and thanks for the input. I would assume this is a corner case and does not represent a typical situation. In this case, one could just send in the zip code for avs match. AVS will return specific codes related to postal code match. Here gain there's a tradeoff, I don't see the point of compromising the UX for 99% to cover the 1% case.

[jacobolus]

For cases where one zip code covers parts of two or three cities, it would also be possible to present some kind of drop-down to select the city. This should still be much faster and less error prone than typing, and it only applies to a tiny fraction of zip codes.

[screature2]

Good point, but as far as I've seen, zipcode seems to identifies locality good enough. If you have the right zipcode it doesn't make a material difference for either sending/receiving mail or for credit card checks.