airpost vs postal service

airpost and postal service both are nouns.

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

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

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

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

airpost (noun)	postal service (noun)
a system of conveying mail by aircraft	the system whereby messages are transmitted via the post office