A true defender of vigilante justice would deliver without a signature.
If you want to get a stranger to do something for you, your best bet is definitely drawing an illustration of them doing their job as Spiderman. Hey, UPS guys are just like everyone else. They appreciate a little recognition for their hard work in the form of cartoon drawings that suggest they are superheroes. Showing them blowing up FedEx is a nice move, since everyone knows about the bloody decades-long battle between these twin forces of good and evil. When will it end, FedEx? When will it end?