Loose tobacco and rolling papers. Even if you don't smoke, this is the most universal possible currency for trade with commercials. Want day 12 cocktail ice? Easy trade for some cigarettes...?
Cigs are not the most universal currency on any GC trip I have been on. Beer, weed, liquor, and ice, have always been good currency in my experience.
May the wind be always at your back,
May the sun shine warm upon your face.