I usually pay by adding the app charge to my AT&T bill.
And how do you do that?
I think it depends where you live, on your carrier in other words. In the US its very easy. When you proceed to pay for an app there is an option to add that amount on your next monthly bill you choose that and you proceed.
You have to have your Data on
Get yourself a Xbox Gift Card and add it to your 'Wallet'.
At a local game store like Gamestop if you're in the US.
Download $10 Xbox Gift Card - Digital Download for Xbox 360 | GameStop