In App purchases are made with iTunes. Refunds may be available by completing the e-mail form available at the iTunes support service. Depending on your country, the e-mail form must be filled within a certain time from receiving the receipt of the In-App Purchase.
The form can be found here: https://getsupport.apple.com/
- Click on "iTunes & Apple Music" > "iTunes Store"
- "Purchases, Billing & Redemption" and select the relevant option