I can't add a card. ZIP is incorrect.
Typically this error means that the address you're inputting is not an exact match to what your bank has on their system. First, make sure the billing info you supply doesn't contain typos. If everything looks correct, but the error is still here, you need to contact your bank to confirm the exact entry of your billing address in the bank's system. Once you have confirmed this with the bank, you can retry entering your billing information.
Please, note that the address you're providing may be correct while your bank has an incorrect entry in its database. Even if there's a difference in a single character, the ZIP check will fail.
If this doesn't help and you're still getting the error, contact us at firstname.lastname@example.org.