Mac – Buying an iPad through an iTunes account

According to Apple's page, it uses the following order for song purchases:

• Song credits.
• Gift certificate, prepaid card, or allowance credits.
• Credit cards.

Thus, it would appear that you cannot charge a credit card without using up your credits or gift certificates first.

However, similar to what @Nick said, you could just open up another account, add the credit card to it, and then gift the items you want to your original account.

Update: Based on my own tests of self-gifting, these are my findings:

• Gifts will deduct money from the credit card on your account; they will not use up song credits nor gift card credit. This means that you do not need to open another account, just add it as a credit card and gift it to yourself.
• You cannot send a gift to the same email address as your iTunes account. This can easily be circumvented by sending it to one of your other email accounts.
• The easiest method of gifting to yourself is to select the print option. It will show you a code which you then use to redeem the song. It will allow you to redeem the code on the same account.

There are a couple of gotchas I'm aware of.

1. If you buy apps on both iTunes accounts, they all get listed together when updates become available, not just the ones on the account you're currently signed into. So if you click 'download all free updates' iTunes will only download the updates for the account which is signed in. This can be confusing because you've told it to download all free updates and it doesn't tell you why it hasn't downloaded the others.
2. If you have an Apple TV and you want to use Home Sharing, then you can only share the content for one account at a time.