ending vs postfix

ending and postfix both are nouns.

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

As nouns, postfix is a hyponym of ending; that is, postfix is a word with a more specific, narrower meaning than ending:

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

Other hyponyms of ending include suffix, inflectional ending, inflectional suffix.

ending (noun)	postfix (noun)
the act of ending something	an affix that is added at the end of the word
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