What are the terms that are actually printed on your coupon, and on their website regarding coupons?

If you think that the terms allow this coupon to be valid, send an physical letter by U.S. Mail that clearly explains the circumstances, and ask for the discount to be applied retroactively. Explain your justification. Send the letter to the office of the CEO; it will get forwarded to a top-tier customer service agent.

If you think you'll get any compensation over and above the value of the coupon, you're probably mistaken....