If possible you'd be better off buying wallet funds, and then purchasing using that after it's all good.

Alternatively Paypal (Especially having a balance) works last i checked, but expect extra hidden fees changing of currency...

Hmmm...

yeah....the wallet stuff didnt work either...already tried that.
Also, nope to paypal...im just not interested.

GOD DANG IT...

not only am i paying crazy prices over steam.....now i cant even buy the sodding games..fml

That would be my first approach to ensure you had funds that had no possible way to be denied. I'm curious what game(s) you're after? Who knows, maybe someone will already have it, expiring codes or purchased gift games that are just floating out there.

I had a problem with my bank card about a year ago, suddenly I wasn't able to purchase any games from GOG. I even called my bank and they told me that GOG is blocking the payments. The solution back then was to remove credit card credentials and input the data anew. Never again I save my credentials on GOG store. You might want to try it and see if it helps.

Check your shopping cart if it contains DLC that you don't own the base game to. That would make the transaction fail.

nope...just have sleeping dogs and metro 2033 redux.