Why do you want to shoy coupons on cart page?
Sorry for my opinion but I do not think that, commercially, it is a good idea to show discount coupons right in the cart page when the customer has already chosen and is willing to buy. It seems to me that showing discounted options is like saying: are you sure you're going to buy this? Maybe this is more convenient for you! I think it is a way of persuading the customer not to finish the purchase with the possibility that he does not buy anything.
It is just only my opinion...
