I have been mystery shopping for a little over a year now. At first I was really skeptical when I learned what Mystery Shopping is. We all have seen ads and received emails for companies that want you to pay to be a mystery shopper. I do not recommend paying a membership to be a mystery shopper. The only thing that I have paid for is a $15 test fee to become certified through The Mystery Shopping Providers Association.
I have shopped for restaurants, banks, credit unions, mall-stores, major retail, pizza delivery, and more. With most companies you do have to pay your expenses up front and you are reimbursed after completing your survey online. Some companies only reimburse for expenses and some pay you a fee. Most of the restaurant ones that I have done just reimburse up to a certain amount - so it's like getting a free date night. Before you accept a job you will know how much it pays and MOST of the details - just do what works for you. I have been paid as little as $1.75 for calling a pizza delivery place up to being reimbursed for a $100 dinner for 2.
Here is a list of Mystery Shop Companies