letter box vs compartment

letter box and compartment both are nouns.

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

As nouns, compartment is a hypernym of letter box; that is, compartment is a word with a broader meaning than letter box:

letter box: a numbered compartment in a post office where mail is put to be called for
compartment: a space into which an area is subdivided

letter box (noun)	compartment (noun)
a private box for delivery of mail	a partitioned section, chamber, or separate room within a larger enclosed area
public box for deposit of mail	a space into which an area is subdivided
a numbered compartment in a post office where mail is put to be called for