The original WordPress invoicing plugin is WP-Invoice, this can be seen at the respective plugins' Stats pages - WP-Invoice was launched on December 31st, 2008. While "Web Invoice" was launched over three months later.
The "developer" of Web Invoice never contacted us, the original developers, asking permission (out of common courtesy, perhaps) to modify our plugin and re-distribute it.
We contacted the"author", asking him to either take down the plugin, or give us noticeable credit, but to no response.
Although we encourage user modifications for personal use, we believe that a public re-release of copied code is questionable in character, especially when the "modifications" are centered around inserting one's lucrative affiliate links.
TwinCitiesTech.com is a respectable company, and we have continued to improve and support WP-Invoice requests throughout the months, as may be seen at our UserVoice page - http://wpinvoice.uservoice.com We've acted upon, and implemented over 30 user-originated suggestions.
So if you are interested in additional modifications to the original plugin, while working with the original developer, feel free to contact us: http://twincitiestech.com/contact-us/