letter box vs Post-Office box

letter box and Post-Office box both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter box	Yes	No	No	No
Post-Office box	Yes	No	No	No

As nouns, letter box and Post-Office box are synonyms defined as:

letter box and Post-Office box: a numbered compartment in a post office where mail is put to be called for

Other synonyms of letter box include call box, PO Box, POB.

letter box (noun)	Post-Office box (noun)
a private box for delivery of mail	a numbered compartment in a post office where mail is put to be called for
public box for deposit of mail
a numbered compartment in a post office where mail is put to be called for