Like one of the earlier commenters said, take your selected items to the checkout counter, try the coupon, and if it works, purchase the item(s). If it doesn't work, leave it at the counter and walk away. When the clerks have to spend a big portion of the day handling 'go-backs', maybe their manager will notice their un-productive day, along with lower than usual sales volumes. Somebody, somewhere 'should' notice.

What the Big Print giveth, the small print taketh away.

"Unlimited" (with a 5gB cap) data plans for cell phones? Same idea.