On the subject of a single coupon code applied
I think the problem then with the current implementation is the view cart function. Everytime you add a product that is coupon enabled (maybe even not coupon enabled and this is where I think there may be a big problem with the code) it shows an input box for coupon code. It leads one to believe that you can use multiple coupon codes during checkout. Yesterday, I actually logged out as user/purchaser, used storemanager to remove a coupon associated with a product, logged back in as user/purchaser, added the product with no coupon associated and it applied a discount and showed the coupon title on the checkout screen. That's a problem.