I need to empty my Apple Store credit balance.

To zero your available credit balance for your AppleID account, you can either purchase something with an equivalent or greater cost than the available balance (any excess being paid by your registered Payment Method) - or contact Apple Support and request that your remaining credit balance is cancelled.

You can contact the Support Team using the Support link at top-right or bottom-left of this page - or using this link:

Official Apple Support

Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044