The Post Office Department advertised its airmail service a number of ways, including posters that were displayed in post offices around the country. In addition to these posters, the National Postal Museum has flags from the postal service's airmail history, including the first U.S. flag carried by airmail and a flag of the U.S. Air Mail service itself.
Flag of the U.S. Aerial Mail Service, 1918.
Add ZIP to airmail and air parcel post
Overnight Air Mail poster
There goes the airmail, is your letter aboard? 10 cents per half once anywhere