Many moons ago, I did this using:
Enom for domain registration
cPanel to create accounts
2CO & another third-party payment gateway
All worked as 'seamlessly' as you would expect considering the three systems involved and that I had to create a new API that hooked into all three (one of the payment providers didn't even have an API so we had to write a wrapper for their online payment "gateway").
In the end, we had a one-click solution where the site, DNS, and billing were taken care of from our main page.
Moral of the story is that you could do this with just a cpanel + WHM server and writing your own "middleware". I would not dismiss this as an option; especially if you can't find something out of the box that does what you want.
Just something to consider as you shop around