Shigofumi (Translation: "shigo" means "after death" and "fumi" means "statement" or "sentence") are letters that the recently deceased write to a desired individual. They are, in essence, an "after death statement". They are sent to said desired individuals by mail carriers like Fumika and Chiaki.
Shigofumi are just like regular letters. They are on white paper in a white envelope with a blank, black stamp.