Since today I can suddenly only pay in USD, why is that?
Bugs.... reload the page!
If that does not help, try exiting GOG and the browser, clear the cache and enter again.
One more: Someone else had luck fixing it by putting a game in their shopping trolley and proceeding to pay then cancelling it.
If you look a bit you will find lots of threads about this problem.
The first thing I did was log out, clear cache and log in again, that didn't help, hence the thread.
Adding something to the cart and going to checkout helped though, thanks!