Q: why have you taken payment from my card when I still have credit in my account

Hello rider 200,

If you have a credit on your account that covers your recent purchases that will be used first.

See How iTunes Store purchases are billed - Apple Support

When you make a purchase, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits. The payment method that you have on file is then charged for any remaining balance.

What you may be seeing is an authorization hold to verify your account, but that is removed after the authorization.

About payment card authorization holds in the iTunes Store - Apple Support

You might see authorization requests on your online statement. These requests aren't actual charges. They are tests to confirm that your payment card account is active and has the funds available to complete your transactions. Authorization holds are removed by your financial institution shortly after your purchase clears. The time it takes to remove authorization requests varies by financial institution.

To review the actual charges on your account, see your purchase history in the iTunes Store.

Best regards.