I don’t use any of the billing companies out there. Instead I make my own invoices which I then send to my clients (private and corporate) when it’s time for a renewal. Then I get paid either by check or via PayPal. I don’t like to split my income with others especially when I can do it myself. I hope this helps.
I am with Harris on not paying for something I can do easily myself. If you have many invoices to write, you can take a look at the Mail Merge function in Microsoft Word. You can create a form (eg. invoice) and a data file and can merge them. It has a bit of a learning curve but may be worth checking out.