I've said this before. A built in billing system would be a bad idea. Most of us bigger hosts provide more then just Plesk. Such as other control panels, game servers, dedicated servers, etc. If we were to integrate billing into Plesk it would mean we would have to do billing in a seperate system for our other sales. Not to mention the way SW-Soft is, we would be restricted to payment proccessors they partner with, domain registration they partnet with, etc Personally Modernbill is the best route to go at this time, and can be found for as low as $12/m with some Modernbill resellers.