Perch Success and Error messages in Cart?

Hi Perchers. Quick question.

Is it possible to use the perch:success and perch:error form tags in the Perch Shop cart? I need to give some feedback in the cart for when a user enters a discount code. At the moment it just either applies the discount, or doesn't. I tried using the standard success tag which work, but then don't display the rest of the form when the cart is updated, just the success message. And I also didn't know what type= to give the error tag for the discount code input, so that doesn't work at all.

If the success and error tags aren't going to work. How would you go about giving feedback on cart actions? I suppose I could do it by checking if the amount in <perch:shop id="total_discounts_formatted"> has changed. Or is there something better?

Thanks