There are some shops that offer a reimbursement that subsidizes the cost of the mystery shopper’s required purchase, but doesn’t cover it in full. That means that you are paying money out of your pocket—money you will not be reimbursed—in order to complete the shop.
Are there ever times when you would find such a shop worthwhile? Perhaps, but you should be aware of the choice you are making when you select such an assignment.
Let me give you an example. I was looking at a fine dining shop that requires purchasing the following: