Make sure you're logged in with your Apple ID in the App Store, iTunes Store or iBooks Store.
It's possible that the amount of money you see in your account isn't up to date, and there are less than the $ 12.99. To make sure you see the right balance, go to one of the stores, tap your Apple ID, select "View Apple ID", and enter your password. If the balance wasn't up to date before (happens sometimes), then it will be now, and you'll find it at the top of the next page (Apple ID Balance).