Undead have been around for hundreds of years. To begin with they were mindless killing machines, but as they ate and expanded they became smarter. Able to talk, communicate.
With so many homeless and jobless zombies wondering the streets. Celestia makes a public announcement, the dead can work retail and live in stores they work in.
Hell, its free labour and no extra homes need to be build. So bosses are over the moon about the idea.
One chapter for each zombie