How to order a "site license" of an Office App?

Hey everyone,
hope I selected the correct community?

I developed an Office 365 app available on Microsoft AppSource.
One of my clients wants to order a "site license" of my app.

I tried it myself but I don't see a possibility to select the amount of licenses resp. to select a "site license".

How is this been done?
(I really hope my client doesn't have to buy 10 single licenses until he reaches the "site license"). 😄

Thanks and best regards
Eric