I noticed that simple Woocommerce settings are not obeyed by this plugin. When I have the Woocommerce setting 'Number of Decimals=0' the error message of the plugin adds still the decimals. Also no Thousand Separator shows up.
It would also be good to hook into the currencies already present in Woocommerce. I have spent some time to add Indonesia Rupiah while that currency is already standard in Woocommerce.
It would be nice to have a build-in error message css maker in the backend. Although it's also possible in the custom.css...
No further comments except that it looks quite good.