Shipment by most economical means

§ 4159. Shipment by most economical means Shipments of official matter other than franked mail shall be sent by the most economical means of transportation practicable. The Postmaster General may refuse to accept official matter for shipment by mail when in his judgment it may be shipped by other means at less expense, or he may provide for its transportation by freight or express, whenever a saving to the Government will result therefrom without detriment to the public service.