I tried your plugin, and it's great for free. Some room for improvement though:
- Instead of creating your own nav item in WP, why not tuck it under the settings menu option, or tools? When many plugins create their own menu item the interface starts to get unwieldy
- Maybe consider removing your styling from the messages. If you don't add your own styling, in most themes the feedback messages will pick up default styles of their given theme. At the moment users have to revert your styles or override them
- Maybe allow users to specify their own feedback messages. The current message I'm getting isn't particularly user friendly 'A maximum quantity of 10 units for the total group may be purchased. The current total for all the products in the group is: 13 units. (Rule ID = 107)'
Hope you find the feedback useful - it's not criticism, just constructive feedback.