Thanks for the question. iTunes purchases are billed to Store Credits first, before Credit Cards, as outlined by the following article:
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.
It also may be that the iTunes Store is prompting to verify your billing information:
When there is a mismatch between the iTunes Store's records and your credit card company's records, you will receive an error message stating that your credit card's security code isn't valid or that your zip code does not match your bank's records.
In most cases, this issue is caused by a formatting error in the billing address that you have provided. You can correct the issue by verifying that the information you have provided is correct, or by changing the format of your address to match the format that your credit card company has on file.