mailing vs posting

mailing and posting both are nouns.

Word	Noun	Adjective	Verb	Adverb
mailing	Yes	No	No	No
posting	Yes	No	No	No

As nouns, mailing and posting are synonyms defined as:

mailing (noun)	posting (noun)
the transmission of a letter	the transmission of a letter
mail sent by a sender at one time	a sign posted in a public place as an advertisement
	(bookkeeping) a listing on the company's records