I do not know a single billing application that can handle this. These are two different monsters. You must use some kind of auth structure in conjunction with a billing application. Ours is called CheddarGetter.com.
What you probably need for a third-party part of this project is a basic member database with some kind of access control to increase the security of your application. The most common way to do this is through a combination of user and password security and session management. IP shielding is also possible, but usually not pragmatic.
There are many frameworks that include some user / author kindness. I'm not sure about .net, but here are a few other options:
:
https://github.com/RailsApps/rails3-mongoid-omniauth
https://github.com/plataformatec/devise
PHP:
http://framework.zend.com/