One of the benefits, and disadvantages, of iTunes is that it saves your billing info. When you've entered your info, you won't need to enter again. This happened to me once when I meant to use a giftcard, and I ended up paying for the media instead. I was irritated.
If you want to clear your billing info, this is how.
1. Turn on your computer, log in if necessary, and start iTunes.
2. Go to the iTunes store.
3. Log in to your account. You'll need to click your e-mail address at the top of the screen.
4. Click EDIT PAYMENT INFORMATION. I don't think you need to see my own options, here.
5. iTunes will list your billing address and the last four digits of your credit card. To wipe your slate clean, click NONE.
6. Click DONE.
7. That should be it! If you want to erase the data provided, that's a good idea, too.