I work with a lot of retail stores and yes, if you add "Coupon Code" to the checkout page some customers will stop and start looking for coupons. There are a couple of options:
a) Don't call it coupon. Call it "offer code" or "referral code" or something that doesn't immediately scream "Discount!"
b) Why try to hide the codes? They will be easy enough to find on Google if they're available so instead, make compelling offers where you provide discounts in exchange for customers purchasing more or taking some other action. Look at rental car web sites. Most offer a "Deals" page where you can see coupon codes because they realized the codes were easily shared anyways.
c) Don't use a text box. Instead create referral links with the code embedded so that only people using the links will get the coupon code. This way, people without codes never see the option to enter one.
d) Check the HTTP referrer when a visitor first arrives and apply coupon codes as needed OR only display the coupon box when you know a customer has come from an advertiser site.
e) Proactively suggest offers that don't require coupon codes at all. For example, when a customer has $97 of items in their cart, display a message box offering a discount if they purchase > $100.
I've never seen codes that make the price go up but I have seen codes that are incompatible with corporate rate codes. For example, Hertz gives an American Express corporate rate but sometimes coupons require a different corporate rate (like AAA) and the coupon + new rate is actually a worse deal than just the original corporate rate.
a) Don't call it coupon. Call it "offer code" or "referral code" or something that doesn't immediately scream "Discount!"
b) Why try to hide the codes? They will be easy enough to find on Google if they're available so instead, make compelling offers where you provide discounts in exchange for customers purchasing more or taking some other action. Look at rental car web sites. Most offer a "Deals" page where you can see coupon codes because they realized the codes were easily shared anyways.
c) Don't use a text box. Instead create referral links with the code embedded so that only people using the links will get the coupon code. This way, people without codes never see the option to enter one.
d) Check the HTTP referrer when a visitor first arrives and apply coupon codes as needed OR only display the coupon box when you know a customer has come from an advertiser site.
e) Proactively suggest offers that don't require coupon codes at all. For example, when a customer has $97 of items in their cart, display a message box offering a discount if they purchase > $100.