free billing managers are seriously rare. most only support pass links, not things like ipn. you might want to look at phpmanager, that supports paypal, but its not free.
██ l 201TB.com 201TB bandwidth as standard in three USA DCs. KVMoIP, auto reboot & OS install all included - Now available in the Netherlands! ██ l udedi.com UK, USA & NL unmetered 100mb & 1GB 100TB premium bandwidth servers
██ l Assiva Ltd UK Shared & Reseller LiteSpeed 10 years of hosting R1Soft