postscript vs addendum

postscript and addendum both are nouns.

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

As nouns, postscript and addendum are synonyms defined as:

postscript and addendum: textual matter that is added onto a publication; usually at the end

Other synonyms of postscript include supplement.

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