lettercard vs postcard

lettercard and postcard both are nouns.

Word	Noun	Adjective	Verb	Adverb
lettercard	Yes	No	No	No
postcard	Yes	No	No	No

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

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

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