post card vs mailing-card

post card and mailing-card both are nouns.

Word	Noun	Adjective	Verb	Adverb
post card	Yes	No	No	No
mailing-card	Yes	No	No	No

As nouns, post card and mailing-card are synonyms defined as:

post card and mailing-card: a card for sending messages by post without an envelope

Other synonyms of post card include postal card, postcard.

post card (noun)	mailing-card (noun)
a card for sending messages by post without an envelope	a card for sending messages by post without an envelope