In case a student has to pay for the course, how does the invoice get to the student? Is it send via e-mail, via regular mail? If it is send via regular mail, how can the administrator get the addresses?
The invoice is sent via email. You can create an invoice template and specify that invoice template in the enrollment template for the paid course that you create. This enrollment template will have an email and address fields. The invoice will be generated and sent to that email address.
The administrator can check the user addresses in All registrations list. A custom report can be generated to get a list of user addresses.
If an administrator need to view the invoices, he can go to User History and choose action Send Invoice.