That's the purpose.

Not with that attitude…

I mean, you can probably stick whatever you want in there.

https://en.wikipedia.org/wiki/Rocket_mail

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 On 8 June 1959, Barbero fired a Regulus cruise missile – its nuclear warhead having earlier been replaced by two Post Office Department mail containers – targeted at the Naval Auxiliary Air Station at Naval Station Mayport in Florida. The Regulus cruise missile was launched with a pair of Aerojet-General 3KS-33,000 solid-rocket boosters. A turbojet engine sustained the long-range cruise flight after the boosters were dropped. Twenty-two minutes after launch, 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, 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 submarine put to sea. At Mayport, the Regulus missile was opened and the mail forwarded to the post office in Jacksonville, Florida, for sorting and routing.

In a just world, they would be.