Exactly. This is why more companies should follow Gumroad's lead: they let you send out a link with the code, but don't display the coupon field in your checkout form.

I think this is the best way to handle coupons. That way you aren't enticing people to leave the check out process once they've committed to buying. You also reward loyal customers with special links. I mean all you have to do is make the coupon field hidden and pass a get var.