Multi-site Billing and Domain Mapping

Hi,

Is there an easy way or a plugin perhaps to have a billing system for site users withing a WPMS network? How about domain mapping that will work with it?

Or should I just use an outside billing system where users can sign up but if I do this, how will I let them create their sites and use domain mapping themselves? Or do I simply need to create the site for them and map it to their domain?