We've recently discovered some issues at our fulfillment house with invalid zip codes and I wanted to run it by you guys to get your thoughts. We are using both FedEx and UPS as shipping carriers with free domestic shipping via UPS. I suspect that the free shipping option is masking errors returned by the carriers.
For example, a customer recently used a zip code of 'CA90045'. My investigation shows that with free domestic shipping turned on there are no errors returned and the only shipping option a customer receives is 'free shipping'.
However, with free shipping turned off the correct error message is displayed in the shipping section as:
FedEx Error: Destination postal code missing or invalid.
UPS Error: The postal code CA900 is invalid for CA United States.
It would certainly be our preference that if errors like this are returned from the carriers that the customer not be allowed to continue with checkout. I thought about trying to throw together a simple zip code validation script but then realized that will get much trickier with our International customers. Any ideas?