@nasM Hosted

There are other options, if you dont like to pay the $20 per month.
You can buy the Standalone version of Billings in the App Store for a one time fee, but there you dont have features like multi user or over the air sync.
Or you can host you own Server by installing Billings Pro Server on a Computer and buying a license for that.

ipodnano about Billings Pro: Time & Invoice, v1.5