You don't need the credit cart if you buy on the console (free or not), it uses your currency balance of your account. And I'm sure they will adapt xbox.com to use the currency on your profile.
You can buy MSP cards in stores with cash and redeem it and it will convert to your currency. Unless they already removed the MSP cards, but I doubt they will remove them before they get their Microsoft Gift Cards in stores. I still see MSP cards in stores these days...
So, there are many methods to buy stuff, you just don,t try enough.