Could you please explain why your 11 year old was not able to use a book to look up a zip code, especially with you there to assist. If it was a 3-4 year old child, certainly, that would be beyond their skills. An 11 year old would be in 5th or 6th grade, and should be capable of comprehending basic reference documents. Your child may need some guidence getting the correct zip code, but there's no reason that they can't look it up.

Every time I've been to the Post Office, they just type in the zip code I provide on the item. It is very possible that they don't have access to look them up.