If anyone else has a similar question – although I wasn’t able to reproduce the issue on my test server, after logging in to Eugenio’s site, I’ve managed to fix it. Generally, our “apply coupon” action was called later than “add to cart” action for some reason. I’m not sure why, as it’s working differently on my test server, but it may be some compatibility issue with some other plugin installed on the server. I ended up adding two new options to the plugin (in v1.3.1): Hook priority and Remove “add to cart” key. Both options are in the “Advanced” settings section of our plugin (in “WooCommerce > Settings > URL Coupons”). So now if you lower the “Hook priority” to 10, and disable the “Remove ‘add to cart’ key” checkbox, it all seems to be working fine.