39 U.S.C. § 5603
The Postal Service may require that mail be transported by freight or express when—
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.
(Pub. L. 91–375, , 84 Stat. 773.)
Section effective , pursuant to Resolution No. 71–9 of the Board of Governors. See section 15(a) of Pub. L. 91–375, set out as a note preceding section 101 of this title.