Announced earlier this year was a sort of joint marketing effort by Coca Cola and Delta. You can find these coupons on cardboard cases of Coke products outside of your base in regular supermarkets.
Included in this joint marketing effort were various promotional oportunities. Coke will test market new products or promote new items. One of the items that was put out earlier this year was a Coke product I believe that was a coffee based protein drink. Certain DL flights were boarded with Planet Java products. Caromeal was one flavor as was Vanilla something. The drinks were quite sweet and had a consistency of motor oil.
Anyway, the coupon is a reciprocal marketing thing.