I don't know if it helps but there is a UK version of Amazon Payments at:
https://payments.amazon.co.uk/homeHowever, as an amazon.co.uk account holder, I find that when I click the "Pay Now" button on my Nabble forum, while I get taken to the .com site, my browser saved username and password appears on the screen.
So I suspect that if you go to the URL above and create an account there you will be able to use it at the page to which you get taken by Nabble.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.