Sun Jun 6, 2004 11:01 AM ET By Holly McKenna ALBANY, N.Y. (Reuters) - Sandra Bachman leads a double life as an undercover agent for the FPI -- Feedback Plus Inc., a company that hires "mystery shoppers" to spy on stores and restaurants to rate their products and service. Bachman is one of thousands of mystery shoppers all over America who can get free meals, go on free vacations and watch free movies -- in addition to being paid for their evaluations. "I love doing it because it's fun...