Human mail
Human mail is the transportation of a person through the postal system, usually as a stowaway. While rare, there have been some reported cases of people attempting to travel through the mail.
More common, at least in popular fiction, is the mailing of a part of a person, often a kidnap victim.
Real occurrences
Henry Box Brown (age 42), an African-American slave from Virginia, successfully escaped in a shipping box sent north to the free state of Pennsylvania in 1849. He was known thereafter as Henry "Box" Brown.
W. Reginald Bray mailed himself within England by ordinary mail in 1900 and then by registered mail in 1903.
Suffragettes Elspeth Douglas McClelland and Daisy Solomon mailed themselves to the then Prime Minister of the United Kingdom, H. H. Asquith at 10 Downing Street on 23 February 1909.
Reg Spiers 1964, mailed himself from Heathrow Airport London, to Perth Airport Western Australia. His 63-hour journey was spent in a box made by fellow British javelin thrower, John McSorley. Spiers spent some time outside his container in the cargo hold of the plane, and suffered from dehydration when he was offloaded onto the tarmac of Bombay Airport. He arrived in Perth undetected and returned home to Adelaide.