Appointments & billings

I am in need of a calendar that includes multiple schedules, appointment scheduling, and billing. This is for a site that has a bunch of personal trainer mini-sites, each site needs a calendar that visitors can schedule time on and prepay for the time. If you'd like to add this to your system, I can work with you on it. If you know of an existing system, I wouldn't mind using it. I don't mind reinventing the wheel if I have to.