lettercard vs post card

lettercard and post card both are nouns.

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

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

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

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