39 U.S.C. § 5603: Transportation of mail by vessel as freight or express

The Postal Service may require that mail be transported by freight or express when—(1) there is no competition on a water route and the rate or compensation asked is excessive; or (2) no proposal is received. A common carrier by water that fails or refuses to transport the mail when required to do so under this section shall be fined not more than $500 for each day of refusal.