Trading the coupon for 1 (maybe 2 if you're generous) trading cards from one of the following games:
Borderlands 2, Binding of Isaac, Brutal Legend, Dust an Elysium Tail, They Bleed Pixels, Dungeons of Dredmor, Saints Row the Third, Hotline Miami, Awesomenauts

Other cards accepted too if you don't have any of the listed ones.