I use a system called resellers panel. It does everything from billing, to upgrades, to customer support for you.
It is kind of like a affiliate program but you can adjust the pricing and services you would like to offer as well as design your own site.
You make a flat fee per sale but the nice thing is each account renews automatically each year and you get paid over and over again.
Check out my site at
DMHP Networks for an example of a template. The program is free and you can check it out at the Resellers Link on the page.