I did some mystery shopping with some reputable firms when I was out of work years ago.
I eventually gave up and told them all to go shove it - for the level of work that was involved, it almost always was not worth it. The last straw was one that had a smartphone app that ate my responses, and therefore I didn't get paid. I didn't find out about it until after the deadline for the assignment.
The shoppers for these companies are very much considered as disposable commodities; the money offered for the assignments was so poor that I literally had to make sure that I wasn't losing money in fuel costs to get there.