Mr. Monk and the Sleeping Suspect - S2-E7
Plot hole: Technically, Brian Babbage's plan would never have worked due to the fact that all outbound mail goes to a main post office from the local post office and then placed through scanners to determine the contents, which would have inevitably exposed the bomb inside.
Answer: The two men are trying to work out a secret deal between them without involving the union, which means the workers' interests aren't being represented and defeats the whole purpose of a union. The mayor would lose labor's support and Cusack's union troubles would just be starting.
Captain Defenestrator