I saw the post by Xanthus below and it made me go check my vouchers. I too have 4 available that expire at the end of the month. So I will just quote what Xanthus said.
"If you have the misfortune to be flying in economy class on a Delta flight this month, I not only feel your pain but can help relieve it. I have four Delta drink vouchers that expire at the end of this month that I can't use. First "I'll take them" with an email address gets them. You can print them out at home and redeem them onboard your flight for beer, wine or a cocktail of your choice. Again, they expire at the end of this month, so you must be flying on Delta in economy class in the next three weeks. Please only request them if you will actually use them. Good luck."