ok I will give the snippet a try. Just for further development: If it only works with updating cart, the button “Update cart” needs to be enabled. It is disabled now, so the user has no chance of updating it. Same for the checkout, there needs to be a button. By now this feature cannot be used as the user has no chance of applying it to the cart. I understand not changing the WooCommerce UX too much, but if you say the pro plugin has this feature and then it is not working… that is not so good. I don’t want to be rude, I think the rest of the plugin is pretty awesome, but it should work if the plugin feature list says so…
Hope I can fix it with the snippet!