According to Cathy Stucker, author of the quite interesting Mystery Shopper's Manual, mystery shopping is a legitimate way to earn extra income. But you do not have to pay anyone to apply for mystery shopper jobs.
An excerpt from Cathy Stucker's Mystery Shoppers Manual 5th edition:
Mystery shoppers are "undercover customers." They are sent to retail stores, hotels, restaurants, banks and many other kinds of businesses to report on the customer experience. Businesses use this information to identify training needs, reward good employees, and even to determine raises and bonuses. As a mystery shopper, you typically work for mystery shopping companies that have been hired by businesses to evaluate their service.
Mystery shopping companies do not charge you to apply. Some people are willing to pay for sites that list available assignments, but you can get free mystery shopper job information directly from the mystery shopping companies that will hire you. The assignments you see listed on the pay sites are often the shops that companies couldn’t fill with the shoppers already in their data bases, so they may be low-paying or in remote areas. That means that the job information you get from the mystery shopping companies for free is often better than what you get when you pay a site claiming they will get you mystery shopper jobs.
The best way to get mystery shopper jobs is to apply to several mystery shopping companies. Make your application complete and professional. You will start getting notices of available assignments. There is a quick start plan and a list of more than 100 legitimate, free mystery shopping sites in The Mystery Shopper's Manual.
Also available is Cathy Stucker's Mystery Shopper's Manual 6th edition. New expanded and updated.