I worked as a mystery shopper for a couple of years, for one company. It was a good way of earning a little extra money, and it was interesting to learn some trade secrets. ( No, I'm not telling!)
I did various assignments, visiting large, established stores. For one chain, I had seven eye tests over about three weeks, sending the spectacles back for inspection each time. Other stores sold baby products, car accessories, and , erm, adult products.
If you find the right firm to work for, it can be handy extra money, but I'm highly suspicious of most adverts. It doesn't seem realistic to do several visits in a day, or make lots of cash. Interesting experience.