It is not free but the price is VERY managable at $35 per lifetime license and it offers control panel integration for cPanel as well as all of the popular payment gateways (including PayPal) and domain registrars.
I recommend ModernBill. It does cost a lot, but you get your money back 10 times. You can teach it to walk your dog, tie your shoes, anything. It takes a long time to teach it all of this, however it is very rewarding. It seemlessly integrates into cPanel/WHM VERY WELL!