USPS vs United States Postal Service

USPS and United States Postal Service both are nouns.

Word	Noun	Adjective	Verb	Adverb
USPS	Yes	No	No	No
United States Postal Service	Yes	No	No	No

As nouns, USPS and United States Postal Service are synonyms defined as:

USPS and United States Postal Service: an independent federal agency that provides mail processing and delivery service for individuals and businesses in the United States

Other synonyms of USPS include US Postal Service.

USPS (noun)	United States Postal Service (noun)
an independent federal agency that provides mail processing and delivery service for individuals and businesses in the United States	an independent federal agency that provides mail processing and delivery service for individuals and businesses in the United States