Hi there, i just figured out what the problem was for me.
1. Go to "www.xbox.com" and login.
2. Go to "My Account" and then "Manage Payment Alternatives" (mines on swedish so dont know if its spelled or phrased that way)
3. Change your gold subscription to the card that you want to use.
4. Make sure you delete all other cards
5. I deleted my cashe on the xbox (dont know if its nessesary though)
6. Try to buy your points / the game that you want.
This worked for me, iv been waiting to get a awnser from microsoft, but as i they say, they only want to make money.
So i got tired of it and started trying different things insted, and i found that this works for me.
Hope this information could help you with your problems.