Coupons are a good way of taking things offline a coupon code can be on a card or in the package with a new purchase. You can use them as part of a referral or affiliate marketing scheme. I wonder though are they turn off in checkout? How often do you go to a checkout see a box for a coupon code and then go off looking for a coupon.
If your thinking of coupons being an incentive to go and buy something then the coupon box at checkout is strange. The customer has already decided to buy something they put it in their basket. They saw the price they are happy to pay it and then you are offering a coupon. The coupon they are entering didn’t make them any more likely to buy anything?
Are you making them more likely to wander off at the point in the process when they were just going to enter their credit card details and complete a purchase. Perhaps its better off earlier in the process or perhaps its just me who acts in that way when faced with a coupon code box. I guess I’ll try running a trial and finding out.