About payment card authorization holds from the App Store or iTunes Store - Apple Support
Apple may test the card for use as a subscription payment method. This is not a fee, but it will have to be cleared to succeed. After that, the charge should be undone or reversed.
I can imagine how bad that is if you didn’t anticipate charges at that time, if you happen to be close to the card limit.
If your payment method is declined in the App Store or iTunes Store - Apple Support
You can't make purchases or update apps until you change or update your payment method.
If you need further assistance, you may want contact the iTunes Store staff.