It's taken awhile, but I am now pretty okay with WP-Invoice.. However, I will admit it really took and while to get there, and it was a pretty bumpy road waiting for certain KEY issues to be acknowledged, resolved and FIXED so I could USE this plugin.. (partial payment issues, invoice body being truncated) I continued to use the now defunct WP3 Invoice plugin because for a very long time I could NOT use WP Invoice. I never asked for a refund for the quote add-on I purchased, but I was very clear that the plugin was not being used and COULD NOT be used, and certainly I SHOULD have asked for a refund..
However, for some reason I was unusually patient and stuck it out.. Let me be clear though that this is not a path I would normally have taken. So I do understand some of the sentiment of these reviewers with regards to WP Invoice support.. I stuck it out because I had a "gut feeling" about this plugin and it's support team.. In particular Denys Kostin & Anton Korotkoff.
The plugin for me NOW works as it should.. It works so well, that I FINALLY converted all my invoices from the now defunct WP3 Invoices plugin over and now use this plugin 100% for my invoices and quotes (quotes is a premium feature). The support response has been better than when I first installed this plugin, though truthfully I haven't need much in a while..