I have store credit, however when I want to buy a book, which costs *less* than my current credit, it only brings up the option to purchase it with my debit card. Can I actually purchase a book on iTunes with store credit?
iBook, iTunes
Normally yes, you can buy anything in the iTunes Store except a gift for someone else. Perhaps something in this thread will help:
https://discussions.apple.com/thread/3516230?tstart=0
You may also be just getting the prompt to verify your payment method even though the iTunes Store wouldn't actually charge your card. That's not unusual if you have a card in your account even though you have a credit balance.
Regards.
Can you purhcase a book on iTunes with a gift card?
