I think the plugin has potential. It is a lightweight alternative to WooCommerce. It includes (EU) taxes and pdf invoicing. On the other hand a lot of options, like creating a well layed out pdf invoice, are hard coded. The EU VAT system is not properly implemented and without proper notice another plugin is installed alongside Upsell Plugin. The plugin seems more like a beta to me.
The biggest bummer for me is a lack of response to my questions and suggestions, over here, by email and even on LinkedIn. It makes me wonder what the plugin is doing here at AppSumo .