This is probably one of those customer protection rules where you have to buy something from the store first... before you can buy from the store.

Valve will eventually realize they're not raking in the dough and investigate all the crashed transaction attempts and then reboot their system.

If you can't wait(could be a few days) then make a support ticket saying their store is broken. Indicate which thing you were trying to purchase but don't give your card number unless they ask for it. Hopefully they can handle it old-school style and charge your card through support.