You should absolutely never have to pay a fee to do mystery shopping. Go to volition.com and that should give you a good start on where you can apply to do mystery shopping.
Also, I was looking at snagAjob, and there was one company on there:
National Healthcare Discount......stay away!!!
I applied for a job with them, and got the call from them....they require you to pay a monthly fee of $18.95 for an access code for your phone, and a $19.99 deposit. I told the recruiter I was not interested. I then researched them online, and found a lot of complaints from people that are already working for them.
Just a heads up!!