There are a number of states with rules regarding coupon/gift certificate/gift card expiration dates. Delta would be covered under these rules in the state where the coupons were issued or received by USPS.

If that state were California, for instance, a coupon is valid forever, absent an expiration date, and must, legally, be redeemed. A gift certificate/card may not expire. It's hard to say whether a free drink coupon is a "coupon" or a "gift certifica