Red/Green Colorblind

I think more coffee shops should give some kind of discount for getting their coffee in a mug rather than a paper cup. Think's mugs are equivalent to a medium paper cup. The cost: exactly the same. I wish they'd charge for a small rather than coerce the in-store drinker to spend more, but I understand. These are tough times. Still, couldn't there be a tiny discount in the interest of environmentalism? 10 cents off, save the polar bears.