Like the subject says, I am looking for a plugin that will list specific products on the cart page to persuade customers to add them to the cart. Ideally, it would pop up on the screen when they click to checkout. Is there an app for that, or is that...

When there is not enough stock for the selected quantity and you try to add it to the cart, my store returns the message "You can't add more [product name] to cart". How can I change that? I am using the theme SPLIT. Thanks!