Integrated Event Registration and Billing CRM?

I recently began leading a division that provides education services. I inherited a registration and billing process that is disjointed and cumbersome to manage. Short-term education classes are created on Constant Contact where we can register and market courses. However, billing estimates, invoicing, receivables are managed on a completely different platform (Free Agent).

I’m looking for something that would essentially integrate event registration and email marketing with billing. Can you please offer some practical, integrated solutions?