I have a form at http://tiny.cc/36xxtw that when it enters the proper coupon code, combined with the proper product, the value = $0.00, at which point, when the user checks out, I do not want them to go to PayPal... yet they still do, and it doesn't show the disounted total, it only grabs the item, pre-coupon.
Try running through the process and see what I mean: choose product Box for $90.00, and no optional services. Type in coupon code FREE0001, which will apply the additional product field, and update the total. I added the total to page three to make sure that the total still shows $0.00. but for some reason, when you click checkout, it does not pass through the negative product.