I've looked at authorize.net specifically their $10/month ARB.
Honestly it's more about the payment itself and less about (what I'm looking for) enriching the ISPconfig basic modulu. To make myself clear here is a short description from 'controlzx': "Full financial management. Orders are placed, invoices are posted & emailed, orders are setup & billed, payments are posted. FULL overage billing system as well. Set limitations to plans that clients purchase and CONTROLzx will bill them the overage charges you specify including but not limited to bandwidth per Mbit, per GB & more".
I wish to say that a much simpler solution will do but it will have to incude:
* basic reporting to see what is operative, on which plan and what is the billing status.
* basic ability to automate bills & reminders.
* code explained so when new verion of ISPconfig comes out we can refresh it accordinly.
BTW: my name is ORI.