When a mans zipper is down for long periods of time, therefore occasionally exposing his "package". This can either be done accidentally or by a tweaked individual... on purpose.
"Dude your package is showing, you sending out airmail or what?"
"Oh damn! Thanks man, good lookin' out!"
"Hey that guys running after her with his junk out!"
"Oh that's cause he's sending his package to her airmail"
"Sick!"
When you send a greeting card (preferably homade with some gay picture of a rabit or some shit on it) filled with your cum. The unsepecting victim then opens the card and sees AIRMAIL BLOWJOB in big red letters, fallowed by a cum smeared picture of three old men sucking eachother off.