Economists draw supply curves as upward sloping. But sometimes, as consumers, we notice that the price per unit goes down if we buy more of something, not up. For example, a liter bottle of Pepsi costs less per ounce than a can of Pepsi, warehouse stores sell breakfast cereal in "jumbo size" for a low per-unit price, and clothing stores offer "buy one, get one free." Why might sellers offer such prices?