box vs letter box

box and letter box both are nouns.

Word	Noun	Adjective	Verb	Adverb
box	Yes	No	Yes	No
letter box	Yes	No	No	No

As nouns, letter box is a hyponym of box; that is, letter box is a word with a more specific, narrower meaning than box:

box (noun)	letter box (noun)
a blow with the hand (usually on the ear)	a private box for delivery of mail
a (usually rectangular) container; may have a lid	public box for deposit of mail
separate partitioned area in a public place for a few people	a numbered compartment in a post office where mail is put to be called for
private area in a theater or grandstand where a small group can watch the performance
the driver's seat on a coach
any one of several designated areas on a ball field where the batter or catcher or coaches are positioned
evergreen shrubs or small trees
the quantity contained in a box
a rectangular drawing
a predicament from which a skillful or graceful escape is impossible