Any purchases you make in Heroes of the Dark are handled via Google Play or the App Store and, as such, Gameloft assumes no responsibility for the process.

Android:
You can find the receipts in your Purchase History on Google Play. Just go to Google Play's support pages here, or you can visit the Google payments center directly here.

iOS:
If you want to review the list of games, apps, subscriptions, music and other content you've bought recently, you can find your purchase history online.

Note that these links will redirect you out of the game.