In the analogous retail situation, with physical coupons, there is no "coupon code" field -- the cashier does not ask you "Do you have any coupons today?" Even worse would be for them to ask "Do you have the $10 off coupon on this dog food?" you'd feel like you lost $10. If you have a coupon, you present it to the cashier, and other customers aren't made aware of the fact that they're not getting the best price.