endorsement vs promo

endorsement and promo both are nouns.

Word	Noun	Adjective	Verb	Adverb
endorsement	Yes	No	No	No
promo	Yes	No	No	No

As nouns, promo is a hypernym of endorsement; that is, promo is a word with a broader meaning than endorsement:

endorsement: a promotional statement (as found on the dust jackets of books)
promo: a message issued in behalf of some product or cause or idea or person or institution

Other hypernyms of endorsement include packaging, promotion, promotional material, publicity.

endorsement (noun)	promo (noun)
the act of endorsing	a message issued in behalf of some product or cause or idea or person or institution
formal and explicit approval
a promotional statement (as found on the dust jackets of books)
a speech seconding a motion
a signature that validates something