PHP Coin is an open source biling system based in PHP. Any good experience in using this script? It used to be designed for webhosting but it has been enhanced for general business billing requirements.
You should have a good background on PHP to tweak some of the errors you may encounter along the way. Or join its community forum so that you can have access to common answers to PHP Coin problems. It's a good program though.
I have used this once when I was looking for a billing system for a web hosting business. It is a good script only that you need some help if you are not well versed on PHP and MYSQL.