You may already know that HostBill works great with Colocation/Dedicated servers offering, allowing to measure and bill for bandwidth usage – more details can be found here: http://hostbillapp.com/features/colocation.php

With 3.9.6 we’re introducing new possibility – your clients will be able to choose their bandwidth billing method, and limits directly from cart! One mouse click in product configuration, and you can set smart product configuration.

Using this method overage rate is calculated automatically, based on pricing client selected during order.
It also allows client to upgrade to higher bandwidth limit anytime during billing cycle!