I've been doing mystery shopping for about 2 years. I never paid for one of those sites, because I heard that everything that was on there, you could find on your own. If you type mystery shopping into yahoo search you will get a ton of companies. You sometimes have to write an essay for them to show that you have grammar skills. Some of my favorites are
www.mysteryshops.com,
www.sassieshop.com/2kern, and
www.sassieshop.com/2bareusa. But I am signed up with a ton of different places. Also, make a new email address to use for them because you get TONS of email reminders and info about shops (rarely is it spam, but still alot). Typically you make around $7 plus reimbursement and you can usually do a few a month depending on your area, so you are not going to make a lot of money, but its nice to go out to dinner on someone else's $. Some are very easy and some are very complicated. You can usually pay by CC or cash. You get paid my paypal, check, or direct deposit, depending on the company. Oh ya, and you always get to request the ones that you want, not randomly assigned. Hopefully this helps get you started!
Sarah