Why payment page shows postal code (zip) error when it is not?

Having worked in the business sector, I can give you an informed opinion on this.. which may or may not answer your question.

The problem you are having is not with Native Instruments, it's with their Payment Provider. Whoever has the job of answering questions for purchases at NI likely has no clue what the Payment Provider is doing. The difficulty you are describing is because most of the businesses in the world verify their billing zip code using Google's zip code list or Dun & Bradstreet's zip code list. If your particular address is listed wrong on the service being used, you're hosed. It's like hitting a brick wall, because everybody assumes that the list is correct.

I just looked at NI's billing process, if you follow the default prompt's to use a credit card, it looks like it goes through Stripe. So you would need to call Stripe and do a little polite "social engineering" to try to get them to help you. I mean, they SHOULD help you, because you're using their service, but you will probably need to try to make them care enough to help you.

I note a button at the top of the selection list to try to process the payment using Link. You could do that, and that would get you away from Stripe's billing zip code verification service.

Personally, I don't like either Payment Provider. If you have a PayPal account, I would just use the PayPal service, and pick whichever payment option you want to use under PayPal. Even if you DON'T have a PayPal account, you can select PayPal, then when it pops up, instead of logging in to PayPal, just click the "Just pay with a credit card" option. Then you can try to make the payment using PayPal's billing zip code verification service.. which is decades older and more reliable than the other two.

Good luck! :)