Feature Request: A simple BUY NOW button for existing customers to buy online courses within a membership site

A way for existing customers to buy more online courses without having to put all their details in again and run the risk of accidentally putting the wrong email. 

When customers buy their first online course they would be sent an email for them to set up their login details for the membership site to access their course. If that same customer wants to then buy one of our other courses we don't want them to fill out a full purchase form or get sent the membership login set up again. A simple purchase now and then their membership area would just update and show them the new online course available.