I have posted this in a couple of places:
IF YOU GET A LARGE CHECK (cashiers or otherwise), DON'T CASH IT! Not even if the bank says it's "real" before you do so.
Inevitably, it WILL get returned and YOU will have to pay the money back to the bank. REAL Mystery Shopping Companies DO NOT send out money and ask shoppers to cash the check, keep a portion of it and send the rest back to them (or anywhere else). PERIOD!
Afterall, they don't know you or, for sure, if you'll even send any money back to them. Why would they send you a REAL check for thousands of dollars and then HOPE that you will send most of it back to them after you've cashed it?!? Why not just keep the check for thousands instead of only a small fee? Only someone who KNOWS that the check is no good would risk sending you "thousands" and HOPE that you will cash it and send them the majority of it back. Afterall, they've got virtually nothing to really lose if you don't!
Good Rule of Thumb: "If it sounds too good to be true, it probably is!".
Don't forget, the "normal" range for fees paid by REAL MSC's for doing a mystery shop is usually only $4-$50. SOME shops will pay higher, but if they are over that, you might want to do some research among other shoppers first. Especially if it's for a "new" company that you haven't already shopped for and received payment as promised.
Sorry for your "situation". Unfortunately, as they say, "Experience is the best teacher. Unfortunately, it is also usually the most expensive!"