I've seen with WP Invoice in the past... very nice. I'm sure this plugin is nice too (downloading it as we speak). For me, these type of plugins are only worth considering in a localhost environment, for 2 reasons, and either of these reasons are good enough not to use them on a live site - 1. SECURITY and 2. SERVER RESOURCES, specially when using a shared hosting account. I'd rather spend my server's resources on plugins that actually have anything to do with the web site.