Thanks for pointing this out, OP (but please refrain from cursing on our forums).
I added the appropriate information to the Website Help page, after consulting it with Destro. It should be enough of a "warning" for future customers about any additional charges which are out of our control.
It wasn't just one bank in one country, but we've gotten only a handful of reports about such international fees, so the notification had to wait in line while Support and the Devs were busy with other things. I hope you guys understand. :)
On the bright side I have never heard from a customer who was charged by GOG.com for more than was stated in his checkout window. :)