I have just has issues with people ordering from Ireland who apperently dont have PostCodes or ZipCodes? And so forcing users to enter one is maybe causing problems?
The best way I have found to get around the Ireland problem is to get them to enter a 0 or something in the postcode field. Irish addresses are so vague sometimes it can be a nightmare getting stuff delivered there at all at times. But apart from that you definitely want to make it a required field. Unless of course you're actually based in Ireland and deliver mainly to Irish customers.
