Hello ginnyfromfairview,
Thanks for posting your question. As I understand it, you have a question about how you can use your Apple ID balance, and I'm happy to assist.
Since the credit balance is attached to your Apple ID rather than one of the stores, you should be able to use the credit balance in either the iTunes Store or App Store interchangeably. This has more information for reference: View your Apple ID balance
"After you add funds, you can use your Apple ID balance to make purchases in the App Store, iTunes Store, or Apple Books, or to buy iCloud Storage. The balance is used for each purchase that you make until there isn't any balance left."
I hope I've answered your question. Take care.