Confess, but tell them you were trying to juggle the staple gun while using both hands to get stuff out of your mailbox and it "went off accidentally." They don't have any reason to think you are acquainted with the tenant for the box above yours. Say something like "Wow, that sure was lucky that I didn't shoot myself, hee hee, that would be hard to explain at the ER", etc. Anybody in public service jobs hears so many idiotic stories they probably won't think yours was anything suspicious
