first-class mail vs 1st-class mail

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

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

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

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

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

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