Newman gets the bright idea to take bottles from New York (where the deposit refund is 5 cents) and return them in Michigan for 10 cents. Kramer stops him quickly and let’s him know that this isn’t a good idea because he’s not thinking about the costs of transporting them. Newman quickly realizes he can get a truck at no cost from the post office, which makes the arbitrage scheme profitable.

The full clip comes from Economics of Seinfeld.