Because all goods compete for the consumer's dollar, an increased availability of other goods at low prices tends to decrease the marginal utility of first-class mail service and hence the demand for such service. A few goods, however, may have the opposite effect, because they complement mail service. For instance, a drop in the price of stationery or envelopes might induce consumers to make greater use of mail service. Complementary goods, however, clearly represent the exceptional case.