PS vs postscript

PS and postscript both are nouns.

Word	Noun	Adjective	Verb	Adverb
PS	Yes	No	No	No
postscript	Yes	No	No	No

As nouns, PS and postscript are synonyms defined as:

PS (noun)	postscript (noun)
a note appended to a letter after the signature	a note appended to a letter after the signature
	textual matter that is added onto a publication; usually at the end