Recently, I received an offer from American Express, where I would receive a statement credit if I used my card on iTunes. Unfortunately, I already had a gift balance on my Apple ID and didn’t expect I’d spend it before the promotion expired.

I used the following steps to charge my card immediately and just add to my account balance:

1. In iTunes, go to the Store menu -> View account (appleID@email.com)…
2. Edit your payment details to the credit card with the promotion.
3. Click the home (icon) button near.
4. Under Quick Links on the right hand side of the window, select Send iTunes Gifts.
5. Enter your own email address, select an amount that qualifies for the promotion, and click Next.
6. Pick a design and click Next.
7. Click Buy Gift to charge your credit card.
8. Look at your email for an iTunes Gift card you can add to your account balance.
9. Switch your credit card back to your original card.

Note: I’m not sure why it isn’t possible to buy/send gifts from the Mac App Store. As far as I know, you have to use iTunes.