Another (very cheap i.e. free!) option is phpCoin. See
http://www.phpcoin.com for download and support. It's not got anywhere near the bells and whistles of WHM Autopilot or ModernBill, but for starting out when you don't want the large outlay for bigger products, it's not too bad. The interface is fairly easy to use once you get used to it, and you can change it to integrate with the rest of the site with a little effort. Good thing is you can use as many or as few of the features as you like.
Kind Regards