postcard vs mailing-card

postcard and mailing-card both are nouns.

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

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

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

Other synonyms of postcard include post card, postal card.

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