airmail vs postal service

airmail and postal service both are nouns.

Word	Noun	Adjective	Verb	Adverb
airmail	Yes	No	Yes	No
postal service	Yes	No	No	No

As nouns, postal service is a hypernym of airmail; that is, postal service is a word with a broader meaning than airmail:

airmail: a system of conveying mail by aircraft
postal service: the system whereby messages are transmitted via the post office

Other hypernyms of airmail include mail, mail service, post.

airmail (noun)	postal service (noun)
a system of conveying mail by aircraft	the system whereby messages are transmitted via the post office
letters and packages that are transported by aircraft

airmail (verb)	postal service (verb)
send or transport by airmail