Anything written in a letter after it is signed

Solution (By Examveda Team)

One word substitute is Postscript.

Posterity: all future generations of people.
Postdiction: explanation after the fact.
Postscript: an additional remark at the end of a letter, after the signature and introduced by ‘PS’.
Corrigendum: a thing to be corrected, typically an error in a printed book.