Certainly telling the company exactly what they want to know is what you should tell them.
If you have a background in this field you should definitely emphasize it. If not, a well written paragraph or two of why your experiences and what you would expect of a business can make just as good an impact. By now we all know that a reliable/dependable, able to follow instrcutions, accurate, well written, detailed analysis coupled with an unbiased an enthusiastic attitude will get you the better assignments.
Mystery shopping is not as simple as most think. Agreed?
Hey and if you can leap tall buildings in a single bound, well, you're my superhero!!!!