Is it possible to set up a paypal payment for just the certificate. So that the user enrols normally without paying, but if they complete the course and want a certificate then they have to pay. Possible?
Well you could do a workaround: at the end of the course they complete, give them a password (which is revealed for example at the end of a feedback activity) and that password is the enrolment key to a course which contains just the certificate and which is entered via paypal.
Could that course that has just the certificate be hidden from the frontend users? So it can be seen on the front?