I knew someone who used to do it but I don’t think she actually earned money from it, it was more like she got to take a friend out for lunch with all expenses paid, things like that. It would be interesting to hear if anyone has earned any money from mystery shopper.
I earn extra money from invigilating exams at our local school. I did it for years where I used to live and am now at a brand new academy within a few minutes walk from home. It’s not bad pay and you are only needed for 5 or 6 weeks a year (depending on their set up) and then only a few hours at a time.