A letter sent via airmail may be called an aerogramme, aerogram, air letter or simply airmail letter.
From the en.wikipedia.org
The letter was postmarked before the landings and sent via airmail, but was timed to arrive too late to be useful.
From the en.wikipedia.org
The choice to send a letter by air is indicated either by a handwritten note on the envelope, by the use of special labels called airmail etiquettes, or by the use of specially-marked envelopes.