Hi The White Snowman,
The article below may be able to answer your question about how your iTunes Gift card balance is used.
Click on the link below to see more details and screenshots.
I've quoted some helpful highlights for you:
When an iTunes Gift Card or iTunes Gift Certificate is redeemed to an iTunes Store account (Apple ID), the credit on the card or certificate is transferred to your account and the card or certificate itself no longer has any value.
Once you've successfully redeemed your card or certificate, the credit amount will appear in your iTunes Store account. Each time you purchase content, iTunes will deduct funds from the credit until it's depleted. You do not need to re-enter your gift card or certificate number to make multiple purchases.
iTunes Gift Cards, Gift Certificates, and unused store credit balances are not redeemable for cash and cannot be returned for a cash refund except as required by law. If your jurisdiction allows for refund of redeemed gift card or gift certificate balances, you may request a refund by contacting iTunes Store Support, selecting a support category and topic, and including the following information in your request detail:
- Apple ID (this is your iTunes Store account name)
- Store credit refund amount
- Current shipping address
Additional information may be requested for verification purposes.
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.
I hope this information helps ....
Have a great day!