Had a friend who was a secret shopper during some downtime between assignments.
He said it SUCKED!
He would go to said store, get help on a "purchase", then have to fill out a multiple page report, turn in the item he "purchased" and wait about 4-5 weeks to be reimbursed for his purchase AND to get paid for his time. He was back at work by the time he was made whole.
I'd sell plasma/sperm first if I was that hard up.