by Jim » Mon Oct 29, 2012 7:08 am
If you have enabled any feature which requires a zip/postal code for the "Ship To" address, such as tax by zip,shipping by UPS, FedEx, USPS, or custom Shipping API, etc. then the shopper must enter that zipcode on the first shopping cart screen so proper tax and shipping charges can be computed. If the shopper changes the "Ship To" address zip code on the screen where the addresses and payment information is entered, then it is necessary to recalculate the tax/shipping costs. Since those calculations are only done on the first shopping cart screen it is necessary for them to return to that screen and enter the proper information.
The only think I could recommend if this is a frequent problem is to add text to the first shopping cart screen to highlight that they must enter the "SHIP TO" zipcode on the screen and not their billing address zip/ postal code.