This article shows how a Proof of payment could look like, for a student downloading a pdf file from Ortrac Payment & registation module

You can customize the different sections of the document, simply by taking information from the registration process. You dont need to send this information, the customers can download the document from the (username & password same used as during the registration process. 

see this article to download pdf

At the bottom of this article the sample document is attached as an pdf file