alternate credit card
how do i use a different credit card for a 1 time purchase without also deducting from my gift card balance?
iMac, OS X Mountain Lion (10.8.4), x 2
You can add a credit card to your account via the instructions on the HT1918 page that you posted from. But your account's balance will automatically be used when purchasing content for yourself (that can't be changed) - you can try gifting the item to yourself (assuming that it's not an in-app purchase), gifting can only be done via a credit card : Send iTunes Gifts.