It depends on the shop.
Some shops do not require a purchase or they require a purchase and a return. In other words, you are keeping nothing and so you are working for a fee, most likely.
Some shops pay no fee but reimburse you for something, such as a retail item, food, etc. Unfortunately, the reimbursement amount for some shops may not be enough to cover even the cheapest things you can buy at those businesses.
Some shops pay a combination of a fee and a reimbursement.
Every shopper will have to figure out for themselves which shops make sense for them.
Good luck.