This was custom developed for the TurboCASH Accounting Project.
I have to revert back to the programmer and just discuss the licencing issues on this.Again ,I myself am not a programmer.But I can find out about your issues.
The Project Leader of TurboCASH which is an open souce accounting package, is very keen on Open-sourcing the Oscommerce Moodle Intergration, if we can overcome the licencing issues.
It has the advantage of being seemless for credit card transactions.We could have done this with EFT as well but we decided that we would land up debt-collecting unpaid courses. So there is a manual log-in for Bank Transfers.
All the programming is done on the OSC side. The basic thought process is to use the model field in OSC as the ID field to compare to the course ID field in moodle,and authenticate with the enrollments
The moodle login is firstname ,oscommerce password.
I believe a user can purchase multiple courses in oscommerce.
But you bring up a very relevant point now, i.e. Can user re-take a course.
We never considered this aspect . But since we are going to sell accreditation courses which users will have to repurchase if they fail ,this will be necessary.