Looks like Peet started a little venture to help pay the bills on his treehouse! Lumber doesn't come by cheap, you know.
(Image below)
(Unless he cut it all down himself, of course, but that raises a few other probably-unnecessary questions. Better not to read into it.)
This is one of the running jokes in the fandom.
But,
I MUST GO THERE ARTHAM LET'S USE THE ROCKET.