I have been a mystery shopper for CoRI for years. Well, I mean, I've been in their system. I don't shop for them much, as they chronically underpay. They are not very good at communication, I will give you that.

That said, I have never had a problem getting a payment from them. Look around - their reputation is for low pay, but not being a scam.

They are also known in mystery shopping circles as "CoRI" (to distinguish them from Certified Reports).

There are plenty of companies out there. Check out some reviews (mostly on big companies, some smaller ones):

http://www.mysteryshopsmart.com/reviews.html