I rather doubt that this ad would fly in the US, but apparently it’s just funny in Norway.
I have to question the effectiveness, though. When I put something in the mail box, I expect it to be delivered to its destination, and I’m not paying postage to subsidize a swarm of traveling gigolos.
Also, I recall that Norway does have a solid conservative political and religious bent (nowhere near as bad as the US, of course) — they banned Monty Python’s Life of Brian, once upon a time. I’m sure someone somewhere was outraged about the postal service advertising heresy.