first-class mail vs 1st class

first-class mail and 1st class both are nouns.

Word	Noun	Adjective	Verb	Adverb
first-class mail	Yes	No	No	No
1st class	Yes	No	No	No

As nouns, first-class mail and 1st class are synonyms defined as:

first-class mail and 1st class: mail that includes letters and postcards and packages sealed against inspection

Other synonyms of first-class mail include 1st-class mail, first class.

first-class mail (noun)	1st class (noun)
mail that includes letters and postcards and packages sealed against inspection	mail that includes letters and postcards and packages sealed against inspection