From the link below:
In 1959 the U.S. Navy submarine USS Barbero assisted the Post Office Department, predecessor to the United States Postal Service (USPS) in its search for faster mail transportation with the only delivery of "Missile Mail". On 8 June 1959, Barbero fired a Regulus cruise missile — its nuclear warhead having earlier been replaced by two Post Office Department mail containers — at the Naval Auxiliary Air Station in Mayport, Florida. Twenty-two minutes later, the missile struck its target.
The USPS had officially established a branch post office on Barbero and delivered some 3000 pieces of mail to it before Barbero left Norfolk, Virginia. The mail consisted entirely of commemorative postal covers addressed to President of the United States Dwight Eisenhower, other government officials, the Postmasters General of all members of the Universal Postal Union, and so on. They contained letters from United States Postmaster General Arthur E. Summerfield. Their postage (four cents domestic, eight cents international) had been cancelled "USS Barbero Jun 8 9.30am 1959" before the boat put to sea. In Mayport, the Regulus was opened and the mail forwarded to the post office in Jacksonville, Florida, for sorting and routing.
Upon witnessing the missile's landing, Summerfield stated, "This peacetime employment of a guided missile for the important and practical purpose of carrying mail, is the first known official use of missiles by any Post Office Department of any nation." Summerfield proclaimed the event to be "of historic significance to the peoples of the entire world", and predicted that "before man reaches the moon, mail will be delivered within hours from New York to California, to Britain, to India or Australia by guided missiles. We stand on the threshold of rocket mail."
Notwithstanding the Postmaster General's enthusiasm, in reality the Department of Defense saw the measure more as a demonstration of U.S. missile capabilities. Experts believe that the cost of using missile mail could never be justified.
4 cents domestic? Not too bad.