Have you ever gone to the grocer and been stopped by the checker just before you hand them or swipe your credit card to ask if you're sure you don't have any coupons for any of the items in your cart?
If you have the checker might have grabbed a copy of the Sunday insert they had sitting there and scanned the $1 off coupon for that one item you happened to pick up but didn't know was actually on sale, or 95% of the time the checker doesn't even bother you about such things. By offering a coupon code box on your checkout page, you're effectively saying "Hey, we might have put a coupon in last Sunday's newspaper, but obviously you aren't special enough to have received it and we aren't going to be nice by offering it to everyone to gain customer loyalty."
If you want to do an affiliate-type discount for a single item, do it through URL parameters stored into a cookie and consumed at checkout. If you want to do a Sunday-coupon-type discount for anyone willing to go find a coupon, offer it to everyone: "Do you have any coupons? Our latest promotions can be found at ..."
Edit: and if doing a Sunday-type coupon, have the box be conditionally shown. If your current promotion is for 10% off the purchase of a Foo widget, don't show it when the customer only has Bar sprockets in their cart. You're just losing conversion at that point.
If you have the checker might have grabbed a copy of the Sunday insert they had sitting there and scanned the $1 off coupon for that one item you happened to pick up but didn't know was actually on sale, or 95% of the time the checker doesn't even bother you about such things. By offering a coupon code box on your checkout page, you're effectively saying "Hey, we might have put a coupon in last Sunday's newspaper, but obviously you aren't special enough to have received it and we aren't going to be nice by offering it to everyone to gain customer loyalty."
If you want to do an affiliate-type discount for a single item, do it through URL parameters stored into a cookie and consumed at checkout. If you want to do a Sunday-coupon-type discount for anyone willing to go find a coupon, offer it to everyone: "Do you have any coupons? Our latest promotions can be found at ..."
Edit: and if doing a Sunday-type coupon, have the box be conditionally shown. If your current promotion is for 10% off the purchase of a Foo widget, don't show it when the customer only has Bar sprockets in their cart. You're just losing conversion at that point.