Forum Moderators: phranque
Not knowing what you're exact needs are I'd recommend checking out the free web tools that USPS offers. I use it to simply validate the mailing address. It wonderfully corrects the zip based on the mailing address and adds the +4 code.
Yes, most countries in the world have a postal number system. In India it's a six digit number, in the UK it's an alpha-numeric mixture of anywhere between 6-8 characters, other countries have a variety of other systems. Very few of those postal systems will have a code that validates to a US zip code check.
It's wise that you are researching this prior to building your site :-)
I must endorse Macro's comments about sites that require a zip code. South African 4 digit codes are often thrown out even though they are correct (for South Africa).
I have lost count of the number of times this sort of thing has made it impossible for me to buy/register for stuff online. Most forms will accept 'na' or 'xx' or 'none' or some variation of that but not all. It drives me crazy.
I also, as a result, often get mail with the word 'none' peculiarly appended to my address.
For postage, however, it's probably still worth making the field compulsory (although preferably not with the five digit formatting), because in so many places post code really is important. You want to be sure whatever you're sending gets there and this is worth a little inconvenience on my part.
In some countries it comes at the very end of the address. In the UK the postcode, like a 9 digit ZIP, identifies a street segment: 1 High St, London WC1A 1AA.
In most of mainland Europe, the postal code is for a whole town or part of city (like a US 5 digit ZIP), and is BEFORE the town: 1 rue Martin, 75001 Paris, France.
So any database that prints city followed by postal code prints garbled addresses in many countries.