It comes as a postcard. About the size of large greeting card. Mine is for Fair Lakes, VA and I live in Rockville, MD also, which leads me to think that this is Washington, DC area promotion only.
"Receive a $20 Best Buy give card with any purchase of $50 or more". Not a mail-in - just show card to cashier and you get a $20 GC. Excludes some brands, special order, clearance, demos, open box items. Valid till end of June.