Rent n units at price y for x hours, where dollars per y-hour is dependent on the number of units n(volume discounts)? Show the user an interface with their current unit-hours and let them draw down against it?
Kidding aside, you can do whatever you want on top of the base application -- it's a standard Rails app, after all. But what you get "out of the archive" is a bog-standard customer pays $x/mo for access to the app which has some limits based on the value of x, and the limits are up to you to implement (aside from the included one, which limits the number of users that can be associated with a customer's account).