I also strongly recommend you look into the Emerald Billing System. It'll
blow you away. I'm sure it'll save you lots of headaches.
It interfaces with RadiusNT to get accounting info.
With it, you can email your invoices and have customers automatically
denied access if they don't pay after a given period. I think there's also
many web based utilities to manage related aspects of the program.
You can also make Email servers authenticate via the user database. And
there's lots more.
Again, it's at http://www.iea-software.com