I have been recently looking for a PHP billing system similar to the one used by a lot of webhosts - WHMCS.
What we provide is an online profile for companies to market themselves to their potential audience. To do so there is a fee of £20/m or £200/y. What I really like about WHMCS is that it manages your entire billing side for you.. the issue you I do have with it is that it's strongly related to web hosting, which we aren't. Also it doesn't seem that flexible to fit into our current system.
Does anyone have any advice or suggestions on any similar PHP systems, or whether WHMCS can be edited enough to do what I need it to do?