One of the earlier forms of commercial aviation involved transportation of the mail. This cover commemorates the first air mail flight by Earle Ovington.
Before crossing the Atlantic, Charles Lindbergh was an early pioneer in Air Mail service. Here is a cover from the first flight opening service in the midwest. I like how Lindbergh included the word "Pilot" next to his name. Little did he know how complicated his life would become only a few years later.