1st-class mail vs first-class mail

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

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

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

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

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

1st-class mail (noun)	first-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