Any purchases you make in Idle Siege are handled via Google Play/AppStore 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 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 here.

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