I only really want to support Paypal as a payment processor, but instead of getting something custom coded I think it's probably better and cheaper to go with an established Billing software.
What do you use? I've heard Lpanel is good, is this the case?
To be 100% honest I think that Modernbill www.moderngigabyte.net/modernbill/index.htm is very over-rated as a billing software, but there isn't really much on the market that can compete with it in terms of features... I have also tried Drams from www.dramsystem.com which was very promising a while ago, but I gave up on it because the developers were constantly releasing "updates" and "upgrades" which were not straight forward to apply...
The pricing is kinda cheeky - having to pay more because you've got more clients. I suppose that doesn't matter too much though when you're starting out
Just had a little look round the web and came across http://mblicenses.com/ - a ModernBill reseller which is verified and has some decent savings to be had!
We use WHMCompleteSolution - www.whmcs.com. We tried them all, MB, CE, WhoisCart but nothing can beat WHMCS. They have great support, regular updates and really listen to what users want. We requested a feature and within 3 weeks it was implemented into a new release. Truly a great system and very easy to integrate into your website.
Problem with WHMCS is that it only supports cPanel systems, this is not a lot of use for anyone like us who are offering multiple platforms. Whois.Cart() allows the customer to write their own gateways (although they produced the Helm plugin with a few days of me providing the API spec, saving me the development time )