You can always find your Google receipts using Google Wallet or the Google Payment Center. Just sign in with the same Google Account you purchased the game with. You will see a list of all your purchases made with that account. Find the right order and click on the link. You will see your receipt.

Apple App Store/iTunes

Open the iTunes Store App and scroll to the bottom and click 'Sign In.'

Click 'Use Existing Apple ID,' then enter the Apple ID and password that you use with the iTunes Store. Click 'OK.'  When you sign in to the iTunes Store, you also automatically sign in to the Music, App Store, iBooks, and Podcasts apps on your device. For more instructions please follow this link.

Find your order and click on the grey arrow behind the order and you will find your receipt. We need this receipt to be of your assistance with billing issues.

Please open the settings of your Facebook account and click Payments. You will find this on the right-hand side of the window. Find the correct payment in the list and click it. You will see your receipt. Or follow this link and check the payments that were made and charged. 

Click download if you want to save it or send it to support.

Log into Steam.

Go to your Steam account history or open the Steam client and click on your username next to the envelope. Choose account details and click "View Purchase History".

Log into your Microsoft Store account.

Go to your Microsoft Store order history or open the Microsoft Store website and click on your username in the top right corner.  Click on "Order History" and you will be able to see all transactions made in the Microsoft Store.