You can access your receipts in your Account Settings. To get there:
1. Login to your account
2. Hover over your name at the top right corner of the homepage
3. Choose "Account Settings" when the menu appears
4. Select Payments:
This will open your order history:
You will be able to view and print all of your invoices from there. Invoices for Coins purchased on the app will include a link back to your Apple purchase history or Google Play purchases for payment details.