What if an attorney at this site said that without an expiration date on the coupon the seller remained liable for the $10 discount? The answer from this site would not be authority for enforcement. You could get advice from an attorney but only at a cost far exceeding $10. Nevertheless this is a contract matter matter and can be handled by many attorneys in general practice.
My comments have been made without discussion. An attorney client relationship has not been established. There may be conflicts which prohibit my providing you with specific legal guidance. Any contact with you beyond these few general words will start with a disclosure of opposing parties so that a conflict check can be made. You should discuss with an attorney.
You have three options: buy the item without the expired coupon, decline to purchase and take your business elsewhere, or hire a lawyer and try to fight it. Hiring a lawyer should cost you several thousand dollars, so that is not really an option. Many businesses accept expired coupons for some brief period, but many don't.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.