This is one of those strips where it isn’t always clear how to transition the audience from the first two panels to the punchline in the third. You just have to lay enough clues and hope the connection is clear.
In public service warning news, please don’t take this as advocation of violence against the postal service. They’re fine men and women. Mr. Mittens is just evil, being a cat and all.
MAILMAN: I would return it to you.
MITTENS: So what if I put the delivery address as the return address? Then you would deliver it for free?
MAILMAN: Maybe, but that's mail fraud. I could send you to prison for that.
BEARD: Looks like your package got returned. What was in it?
KLOWNUS: Dead mailman.