Q: I just got an IPod Nano, and I have entered codes for 3 $10 gift cards. It shows that I have a balance of $30 available but keeps asking me to enter billing information when I try to purchase a song

Hello KarlafromMN,

Thanks for using Apple Support Communities.

For more information, take a look at:

Using an existing Apple ID with the iTunes Store, Mac App Store, and iBooks Store

http://support.apple.com/kb/HT2589

Use your Apple ID with the iTunes Store

1. Open the latest version of iTunes.
2. Choose Store > Sign In.
3. Enter your Apple ID and password and click Sign In.
4. Click Review when asked to review your information.
5. Enter your billing information.
6. Click Continue after you enter your credit card and billing information. You can remove or edit your credit card information at any time.

iTunes Store: How iTunes Store purchases are billed

http://support.apple.com/kb/HT5582

Order of iTunes billing methods

When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.

iTunes Store: Changing your payment information

http://support.apple.com/kb/HT1918

Change your payment information on a Mac or PC

1. Click to open your account in iTunes (you may need to sign in with your Apple ID), or follow these steps:
   1. Open iTunes and click iTunes Store.
   2. Click Sign In and enter your Apple ID.
      
      
   3. Click your Apple ID and select Account.
      
      
2. From the Account Information page, click Edit to the right of Payment Type.
   
   You can see the payment methods that the iTunes Store accepts in the Payment Type section. If you want to remove the payment method, select None in the Payment Type section. See what to do if None isn't showing or you can't select it.
3. Change your information and click Done.
   After you change your payment information, the iTunes Store places an authorization hold on your credit card.

Have a nice day,

Mario