lettercard vs mailing-card

lettercard and mailing-card both are nouns.

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

As nouns, mailing-card is a hypernym of lettercard; that is, mailing-card is a word with a broader meaning than lettercard:

Other hypernyms of lettercard include post card, postal card, postcard.

lettercard (noun)	mailing-card (noun)
a postcard that folds so the message is inside	a card for sending messages by post without an envelope