A female employee of the Marc Jacobs store on Mercer Street received a package from California that contained cocaine -- but she called the cops because she thought it was anthrax. The FBI was called to the scene, and after investigating the package, concluded it was a local matter and turned the case over to the NYPD. After running a test, cops determined the substance was cocaine, and took the employee in for questioning. "It appears to have been a huge mix-up," says our source. "The girl has no idea why she was sent it. The cops are looking into whether it was intended for someone else in the building. But she was not happy with how she was treated by the police and is considering filing charges." The employee was not charged. A rep for Marc Jacobs had no comment, while a manager at the store hung up. -NY Post