I could kind of imagine it in a world where the concept of coupons was invented on the web and e-commerce "coupon code" fields were the only place people encountered it.
But of course that's not the world we live in. Our mailboxes get stuffed full of coupons. We get them on our receipts. We get them in our e-mail. We get them in newspapers and magazines and even attached to the very products we're buying.
We're all used to the concept. Roughly nobody is getting upset at the idea that there might be some coupon out there that they don't happen to have at the moment.
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.
Internet coupons are different from physical coupons. You can potentially find internet coupons by searching the web for 20 minutes, while if you're at the checkout at the grocery store, you're not going to go all the way home, look through the trash to find some coupons that were mailed to you a week prior and promptly thrown out.
The difference is the knowledge that there is a coupon for the particular item I'm buying, and the (in)convenience of finding that coupon.
I'm convinced that many, many people are "upset" about the idea that they're missing out on coupons. My wife is one of those people, she can't buy anything on the internet without searching her coupon sites first. And I know a lot of people like her. For me, it takes a conscious decision that "this is not worth it, I'll just pay whatever the price is and save the time and energy".
But of course that's not the world we live in. Our mailboxes get stuffed full of coupons. We get them on our receipts. We get them in our e-mail. We get them in newspapers and magazines and even attached to the very products we're buying.
Yes, and I am sick to the back teeth of this crap in my mailbox and encouraging me to keep bits of paper in my wallet yadda yadda. Coupons suck ass and I make a point of preferring retailers that don't use them.
BTW this isn't 'the world that we live in.' This is a largely American phenomenon. In many European countries you can tell the mail carrier that you don't accept junk mail.
But of course that's not the world we live in. Our mailboxes get stuffed full of coupons. We get them on our receipts. We get them in our e-mail. We get them in newspapers and magazines and even attached to the very products we're buying.
We're all used to the concept. Roughly nobody is getting upset at the idea that there might be some coupon out there that they don't happen to have at the moment.