Let me start by saying that I am new to mystery shopping having only completed a few shops so far.  So I'm no expert and probably below the novice level.  But there are a few things I've noticed so far and that is there are some scams out there.  One of the scams is a company which charges you a monthly fee. I found the best way to avoid being scammed is to only sign up for companies that are members of the Mystery Shopping Providers Association (MSPA) (http://mysteryshop.org/shoppers/).  There are scores of legitimate companies that perform mystery shops along a broad spectrum of industries.  And it's relatively easy to sign up for any of the companies and none of the companies listed on the MSPA website charge a fee for shopping for them.

And because you pay no fee, you get to keep all the money you're paid.  But don't think being a mystery shopper will make you rich.  It won't.  Most mystery shops pay around $10.00, sometimes more, sometimes less.  You have to provide your own transportation to perform the shop.  Then figure out the time involved in searching for the shop, reading how to perform the shop, traveling to the shopping location, spending roughly an hour performing the shop, and completing the shopping report.  You'll find that your pay is really a lot less than minimum wage.  But if you have free time and are not doing anything else and want to make a few extra bucks, mystery shopping could be the ticket.