Stamps are technically legal currency, and prisoners aren't allower to have money, but they are allowed to have stamps so that they can correspond with family and Lawyers.

I have no clue as to the rest of Chang's eccentricities, but brushing your teeth with salt is something my great grandmother used to do, while my grandmother used to use dry baking soda. For my great grandmother I think it was something to do with toothpaste not being readily available in very rural Depression Era…