I try to buy at GOG and the order don't work. What's up with that

Do you have Paypal? You can try using it. There are several ways to use Paypal. You can link your card directly to your Paypal account or top up funds through your bank account. One issue regarding purchases, is that US banks tend to not allow international transactions (requires calling your bank to lift these restrictions). Feel free to try other payment methods to see if they work. Also, check your cart to make sure you don't have any DLC items for games that you don't own. The normal order button will be grayed out if you do.

If all else fails, contact support. They will help you.

https://support.gog.com/hc/en-us/categories/201552965-Orders-and-Payments

Is that perhaps something that was traditionally the case but not so much anymore, or dependent upon transaction amount? I ask because I have completed many international transactions since 2014 without issue. Most were with Fanatical née Bundle Stars or GOG, and using a credit card from either U.S. Bank or Chase Bank.

I did have "International Processing Fees" applied, but Chase dropped those fees not long ago.

How are you trying to pay? Steam gift cards or something? :)