air letter vs airmail letter

air letter and airmail letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
air letter	Yes	No	No	No
airmail letter	Yes	No	No	No

As nouns, air letter and airmail letter are synonyms defined as:

Other synonyms of air letter include aerogram, aerogramme.

air letter (noun)	airmail letter (noun)
a letter sent by air mail	a letter sent by air mail