You can set up a password protected area on your website. My webmaster is working on that for me as we speak (
or as I type 
) . Users would be able to access the course after paying and would then be able to access the course with a USER-ID and password. You can even set a time limit for accessing the course---for instance three months , a year etc.
His services are very reasonably priced.
Let me know if you would like his contact informtion