If you want to refund, you must contact with store where you make purchase. If you purchase on the Apple AppStore or iTunes app use that link to Apple website and read how make it: https://support.apple.com/en-us/HT204084 We doesn't processing your card and haven't access to payment i...
If you need information about you Payment / Receipts ( example: when you was make purchase app ), use that official Apple help instruction: https://support.apple.com/en-us/HT204084
If you have the promo code, you can use that as a free download of the app or unlock In-App Purchase for free. Official instruction from Apple, how to redeem the promo codes, you can read here: https://support.apple.com/en-us/HT201209