You can access your invoices in your Account Settings. To get there:
- Login to your account
- Hover over your name at the top right corner of the homepage
- Choose "Account Settings" when the menu appears
- Click "Order History" at the top of the following page
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 your iTunes purchase for payment details.