I've never seen that one, but did some searching and found a post where someone was trying to buy the "full time mayor" pack. My best guess would be that it was an advertisement for an upgrade pack during a release of one of the add-ons, most probably the digital download deluxe version.
They've changed their site quite a bit since release (all of my saved forum post links are invalid), so it was probably a link to the store, or a blog post that is no longer at the same place. Too bad - sounded intriguing 