Well, I am not sure if I would try to use the RBS environment for this purpose or not. We don't do this on our website, but rather have a CRM system that we wrote (using StrataFrame) that has RBS security within the WinForms side of the application, but the web side doesn't use RBS as it realy doesn't merit an RBS type of implementation.
We have a customers table and then customer sites and products beneath that. We then have flags and triggers associated with the customer record that indicates whether or not they are trial or unlimited users (amongst several other account types) and then we have the products that they are either trying out or that they have purchased. They can access their account regardless of their state (i.e. to change address information, purchase a new license, upgrade, etc.). Their active/subscribed products determine whether or not they can access certain downloads and components.
This would be no different on your side as you would just limit them as to what they could access on your site. For example, if they do not have an active subscription and try to get someplace that is for subscribers only, then you could redirect them to a page denoted this and even have a link saying, "Click here to buy more time" kind of thing.
Trying to use RBS users and permissions could be frustrating and limiting as the RBS is not designed to be a licensing tool, which you have the need within your application.
So I would take the CRM WinForms approach, first, and create your maintenance and application structure on the WinForms where you can maintain and update customer accounts. Then you can implement the web side which allows users to create accounts, purchase products, etc. So this sounds like it should really b ea two sided application with both the administrative console (which would be WinForms) and then the client side where they access their products and accounts from the web.