"Send cards to the address below:
Holiday Mail For Heroes
P.O. Box 5456
Capitol Heights, MD 20791-5456
All cards must be postmarked no later than Friday, December 9th. Cards postmarked after this date will unfortunately be returned to the sender. This deadline ensures enough time to sort and distribute cards before the holidays.
After the mailbox closes, the cards we received will be screened for hazardous materials by Pitney Bowes and then reviewed by Red Cross volunteers working around the country."
(from the Red Cross site)
Check the bottom of the page for how to contact the Red Cross via email, Twitter and Facebook.