Once you have made your purchase, you have the ability to generate your own invoice from inside your account. Make sure you are logged into the Rough Pixels website.
Go to: Members > Purchase History
Then, click the Purchases side column menu link, and then under the Payment section, click the View Invoice link for the Payment ID.
This will take you to a page with your invoice that you can print off or download as a PDF file.
Your invoice will look similar to the above screenshot. Once you are done printing or converting it to a PDF file that you can download, there is a BACK button to return you to the Account Details page.