absolutely not worth your time over a real job great as a supplement
the work itself is fine. you need a good memory to remember stuff like how many other customers there are, the names of employees, what they have advertised, etc then you fill out a fairly large questionaire and then write 400-800 words detailing the experience. each shop I did was worth around $100 ish, between what I bought (they specify what you need to buy usually, sometimes its a free for all, but they cover the costs) plus the payment itself
you never work set hours, "assignments" are posted on a first come first serve basis usually. i live in a non-competitive city so i usually was able to get the good jobs when they showed up. in a place like new york, good luck lol
the best secret shops were for cinemas since you got a free movie + popcorn and drink, and get paid on top of that eb games was good as well since it was usually any game at a large discount, + payment
sounds like a decent first job, much more interesting than the typical