As all payments in Android are handled by Google Play, you should send your refund request directly to Google Support:
- Open Google Play account page from PC: https://play.google.com/store/account
- Click on the three vertical points icon in ACTION column and choose "Report a problem" for the app you want to be refunded.
- Select an option in drop down menu "I'd like to request a refund".
- Fill out the reason why you want a refund and click on the "Send Report" button.