Jim there are alot of programs out there to handle what you need for a hosting business. Which one is best for you will depend a little on how much you want to spend and what features you need.

I'm currently using one called clientexec which is working fine for me though it does lack a few features I would have liked. I think it cost $125

Another comparable priced billing manager is whmautopilot which has a few more features and works especially well if you use whm/cpanel on your server.

modernbill probably has the most features and also the highest price tag. I have heard it's harder to setup, but once setup people seem to find it easy to use.

If you go to webhosting talk forum and search for billing manager there's a lot of posts on many more with a lot and I mean a lot of opinions. I found the best thing to do was visit the sites for each. Most have a demo you can explore.