Mint Panel comes complete with a fully featured billing system.
Set up recurring payments with your clients.
Automatically suspends services if payments are missed.
Custom invoices can also be setup by administrators or resellers for miscellaneous items.
Multiple payment gateways supported with support for custom gateways to be added.
Mint Panel also allows you to bill your customers however you want! Whether it be a conventional monthly recurring invoice or a "pay-as-you-go" approach or both! Mint Panel allows you to setup and customise your own billing models. For example, Using Mint Panel's billing system I can invoice a customer on a monthly basis with flat hosting cost of $3.00 and can also then charge them by the following with a unit multiplier of your choice.
Support for clan pay so multiple users can contribute toward the cost of services
Mint Panel allows you to create your own payment gateway plug ins with its extensive API support.