If you really want to be out of the facility regardless of what they are willing to do for you, I agree with the post about discussing this in person with your boss. Not calling or showing up is the easiest way to burn your bridges (even if you don't intend to ever work there again). Meet your boss and tell him/her the real reason you want to leave. If you don't mind staying if they extend your orientation, you should tell that to your boss too. It's totally ok and professional to ask for a longer orientation if you don't feel ready. Good luck!!