I just had a customer complain that when he put in the zip+4 in the
shipping address
1) I entered a 5 digit zip code on the page where shipping costs were
calculated. When I got to the next page where credit card data was to be
entered, I entered the same zip code in 9-digit zip+4 format as part of my
billing address. The site then proceeded to reject that zip code as not
matching the one used for shipping calculations.
2) I attempted to correct that problem by changing the billing address zip
to 5-digit format. The site was unable to recognize the change and still
rejected it.
It seems like an easy fix to compare the first 5 digits and then accept it.
Best regards
mark