Calling your customers idiots doesn't say much for you, does it?
Seriously -- everyone wants something for nothing and ecommerce is one fo the few places where a customer can buy something, an hour later discover there are coupons "out there" and want a credit.
Your choices:
1. take care of these high maintenance customers and make them happy;
2. make the checkout more complicated
If a customer doesn't ask for the coupon before we ship, we'll usually offer a gift card toward their next order. Yes, it's a bit of work (15 seconds) and we lose some margin, and we keep customers for life.
See: Real Company Shit, by Wil Reynolds (scroll down)
http://www.seomoz.org/blog/2012-mozcon-videos-are-here