lettercard vs postal card

lettercard and postal card both are nouns.

Word	Noun	Adjective	Verb	Adverb
lettercard	Yes	No	No	No
postal card	Yes	No	No	No

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

Other hypernyms of lettercard include mailing-card, post card, postcard.

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