You state that this plugin has been tested with WooCommerce 3.x, however I get an unmanageable number of notices when applying and using coupon codes, such as:
Notice: id was called incorrectly. Coupon properties should not be accessed directly...
As far as I understand, in WC3 directly accessing object properties such as
$coupon->id etc. is deprecated, hence the php Notices. On my sites I have error_reporting(E_ALL) in order to log all possible issues, which helps a lot to track down any issues such as this one.
Although the plugin works for the time being, it is not actually updated to follow WC3 best practices, and I find it difficult to wade through the huge number of log messages generated by the plugin.
Are you planning to update the plugin properly and if so, when can we expect it to be released?