airmail letter vs aerogramme

airmail letter and aerogramme both are nouns.

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

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

Other synonyms of airmail letter include aerogram, air letter.

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