There are real "mystery shopper" jobs out there, but as far as them being a "deal" that could really be debated. Currently there are many people in your situation who are turning to shopping, so both the fees for shops are down, the number of shops available is down (many clients have gone out of business, are closing locations or have stopped their shopping program) and the competition for work is fierce.
The standard job pays under $15 and likely at least part of that is a reimbursement for a purchase you must make to complete the job. The standard company will take 4-6 weeks to pay and reimburse you.