fun fact. in federal prisons (and some portion of state prisons), books of stamps are essentially $5 bills. It's common to see people with huge wads of stamps in their pockets much like you would with cash. a few years ago, it became much more difficult to convert stamps back to cash, so a few companies popped up that would accept stamps in the mail for some value on the dollar and books of stamps are the currency of (most) US prisons to this day.