How do I refund App Store credit

Use this form and make your request. Apple refunds accidental purchases all the time!

Apple/iTunes Support form -

https://www.apple.com/emea/support/itunes/contact.html

ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 3 cellular, watchOS 5.3; iPad Air 2 & iPhone 8+, iOS 12.4; TV 4th Gen, tvOS 12.4;  HomePods, audioOS 12.4 and Airport Express w/ Airplay 2

I don’t think that you can unless it is required by law where you live. See the Apple Recommended response in here by King Penguin.

How can I get a refund for funds added to my Apple ID?

There is also this;

https://support.apple.com/en-us/HT204084

It would be better if you can please post a screenshot, in order to understand the phenomenon better.

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you.