I can see this issue has been raised before and it was just a waiting game to get it fixed - but the issue reported was a few years old.
It looks like we're having the same problem now.
We have uploaded the app and trying to pay the fee. I have clicked on the "Buy now" button from at least 3/4 different places and get the dialog displaying but constantly loading (see screenshot). As far as I can see there are no JS errors.
It's also impossible to find out exactly where I should be correctly reporting this problem - is this forum the correct place? Apologies if it's not!