I have a few special shopping cart rules which customer can apply through an activation URL. This is working fine, but I need to be able to display a message if the customer removes a product or changes the quantity in a way that the shopping cart rule is no longer valid. What's the best way of achieving this? Is there an event to listen to?
Steps should be as follows
Customer gets an activation link, which will add the products with quantity needed to the cart and apply the shopping cart price rule.
If the customer changes the quantity or remove a product, a message needs to show letting the customer know the Promotion Code is no longer valid or something similar because the X product is removed etc.