Originally Posted by SarahKay
What do you mean by billing system? Are you selling something that would require rebilling overtime or is it one-time purchases? Or do you mean you are looking for a shopping cart for your website?
Paypal is not free, but it is well-known and buyer-safe. I have recently been looking at different billing systems and have decided to stick with this one because of it's status as an industry standard.
PayPal is who I first thought of as well. It depends on what exactly you're looking for, but they're a fairly simple setup and a lot of people are familiar with them. Unless they've changed recently, there's not an 'upfront' cost, but they do take fees out of payments you receive.