Postmark Ganymede is a science fiction short story by Robert Silverberg, first published in 1956. 

Consider the poor mailman of the future. To "sleet and snow and dead of night"--things that must not keep him from his appointed rounds--will be added, sub-zero void, meteors, and planets that won't stay put. Maybe he'll decide that for six cents an ounce it just ain't worth it.

Postmark Ganymede was first published in Amazing Stories September 1957.

Robert Silverberg (born 1935) is a prolific American author, best known for writing science fiction. He is a multiple winner of both Hugo and Nebula Awards, a member of the Science Fiction and Fantasy Hall of Fame, and a Grand Master of SF.