I'm wondering if anyone can help me find a good billing solution.

I'm looking for a simple product that just does billing for hosting. I don't need tons of bells and whistles. I do need an API, and would prefer to not have to run any software. Just log into a web admin interface.

I'd prefer to be billed by the account, not a percentage, or flat fee.

Any suggestions?

What exactly are you trying to accomplish?

Try WHBilling at: http://www.webhostingbilling.com/

It's much better than an outsourced billing solution.. You get more control over all aspects.. :)