Both Uses

postscript
in
Flowers for Algernon -- Short Story
(Edited)

And P.S. please tell Dr Nemur not to be such a grouch when pepul Iaff at him and he woud have more frends.
p. 22.7 *

P.S. = an abbreviation for "postscript" that is placed at the end of a letter (after the signature) to indicate that an additional note follows
P.P.S. Please if you get a chanse put some flowrs on Algernons grave in the bak yard ….
p. 22.9

Definitions:

(1) (postscript)
a note added to the end of a letter after the signature -- usually preceded with "PS:" or "P.S."

or:

any written or spoken addition added after the end of something
(2) (meaning too rare to warrant focus)
PS can also be used as an abbreviation for public school as in PS-101. Of course, p.s. can also be someone's initials or another abbreviation. Adobe PostScript is a language used to describe how a page should be rendered by a computer.