All in-app purchases in Little Big Robots are processed by Google through Google Play Store.
As such, you can view detailed information about purchases made through the game in the purchase history of your Google account:
- Go to payments.google.com and log in with your Google account;
- Select 'Activity';
- Choose an entry in the purchase history to view an e-receipt.
More ways to view your order history are available, you can read about them here.