When customers use their credit cards to check out on your Brandibble site, one of the steps in place to prevent fraud is to ask them for their zip code. It's not a step we can remove from the site since it's an important security measure.
When customers receive an error message on the checkout page stating their zip code is incorrect, it is most likely because it does not match the zip code associated with the billing address on the card. It's possible that their address recently changed or they're using a company card and are unaware of the zip code on that address.
If they use the same card on sites that have already saved it or that don’t ask for zip codes, they may not be aware of what it is or that it might have changed. It’s also possible that certain cards require a full address to be submitted for verification (rather than just a zip code), which is an extra security feature that could likely be relaxed. This is pretty rare, however.
The best way to solve this issue is to ask the customer to reach out to the issuing bank to see why their card is being rejected. That will help them get to the bottom of it.