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Our friends at Apple suggest that you contact them directly with any redemption issues:
How can I check the balance on my iTunes gift card?
Check your Apple iTunes gift card balance at the iTunes store. Create an account at iTunes if you don't have one already. In the Quick Links box on the right-hand side look for the "Redeem" link. Enter in the gift card information and your updated balance will appear near the top of iTunes when you check it.
How can I Redeem iTunes Gift Card on my iPhone?
You have plenty of choices when it comes to where to redeem your iTunes gift card. Ultimately it won’t really matter where you choose. But you can redeem your iTunes gift card in the App Store, the iTunes Store, or the iBooks Store on iPhone. You can also redeem your iTunes gift card in the Mac App Store on your Macbook. Regardless of where you redeem your iTunes gift card, you’ll be able to use the balance in any of the listed stores so long as the same Apple ID is associated with all of them. So, to redeem an iTunes gift card:
Open one of the stores listed above (iTunes Store, App Store, iBooks Store, etc.) on iPhone.
Scroll down to the very bottom of the page and select Redeem.
Tap Use Camera and scan your iTunes gift card with your iPhone’s camera,
OR Tap “You can also enter the code manually,” to do so. Use the the code on the back of the gift card.
The card's balance will then be added to your Apple ID account and will appear in Wallet via your iTunes Pass.
When you are ready to make a purchase, the amount due will automatically be deducted from your iTunes balance. Any remaining amount due will be charged to the credit card you have associated with your Apple ID.