postfix vs ending

postfix and ending both are nouns.

Word	Noun	Adjective	Verb	Adverb
postfix	Yes	No	No	No
ending	Yes	No	No	No

As nouns, ending is a hypernym of postfix; that is, ending is a word with a broader meaning than postfix:

postfix: an affix that is added at the end of the word
ending: the end of a word (a suffix or inflectional ending or final morpheme)

Other hypernyms of postfix include affix, termination.

postfix (noun)	ending (noun)
an affix that is added at the end of the word	the act of ending something
	the end of a word (a suffix or inflectional ending or final morpheme)
	the last section of a communication
	event whose occurrence ends something
	the point in time at which something ends