Airmail etiquette

An airmail etiquette, often shortened to just etiquette, is a label used to indicate that a letter is to be sent by airmail. The term is from French "label, sticker" (cognate to stick), from which also comes the English word etiquette "rules of behavior".

Because the etiquettes are just instructions to postal clerks, and have no monetary value, their printing and distribution need not be as carefully controlled as for postage stamps, and most are privately produced. The usual design is a plain blue oblong, with the phrases "AIR MAIL" and/or "PAR AVION" in white letters. Airlines and hotels have also produced etiquettes, some quite attractive.

The airmail etiquette may be omitted if airmail stamps are used on the letter, and in some cases even this is not necessary if a country sends out all its foreign mail by air. In some countries, such as the United Kingdom, you may simply write "PAR AVION -- BY AIR MAIL" on the envelope, even though etiquettes are available free from post offices.

...
Wikipedia