We are using H-Sphere for 3 years now. The integrated administration interface of Hshpere provides resellers a the widest array of tools to manage their hosting enterprise. It use the automated billing system to automatically invoice and charge your customers.

In Hsphere ? you are getting billing as a part of it and completely integrated. You can custom bill for additional services (recurring, one time, any other frequency) and so on...Like

* Variety of payment methods
* Setup, recurrent, and overlimit pricing
* Taxes
* Tax exemption
* Email invoicing
* Reports
* Billing history
* Suspending debtors
* Discounts for different billing periods
* Promotions for plans
* Credit limit changeable on user-by-user basis
* Multiple billing profiles
* Refunds

I think H-sphere is scalable.