Custom Cart Code to be comptible with coupon codes

Hi

FAO Ash. Please see this thread

https://premium.wpmudev.org/forums/topic/add-shopping-cart-items-amazon-style-to-my-main-navigation-menu?replies=38#post-1119949

For some custom code I'm running to have a shopping cart working in my header. It's great but it doesn't quite work with coupon codes.

Please see attached screencast to show the issue. It works on the check out page but not any of my other pages

We ideally need the floating cart which is part of Marketpress now to function the same and apply coupon codes to the cart. I made a feature request but maybe you can help here too.
Kind Regards
Kieran