Though it's possible for shoppers to make as much as $40,000 annually on a full-time basis, it's rare and requires always being on the road, Mershimer said.
Instead, aspiring shoppers should look at it as a fun side gig that might offer only reimbursement for an experience or item, said Cathy Stucker, who has been mystery shopping for 20 years and is author of "The Mystery Shopper's Manual."
"Not only have we gotten to eat for free, … I have been fully or partially reimbursed for massages, eyeglasses, groceries, cellphones and much more," she said.
Mystery shopping allowed Indianapolis-based Cherie Lowe, author of "Slaying the Debt Dragon," to go on dates with her husband while paying down $127,000 in debt.
"We typically visited chain restaurants, and those meals were completely covered, plus a little cash. It was a great way for us to supplement a very slim budget for dining out," she said.
Just don't expect to be paid to visit your favorite establishments too frequently.
"Most of the shops have restrictions on how often you can call or dine, … so you may have to wait up to six months before you can return to do a shop you like," said St. Petersburg, Florida-based Jen Smith, who shopped for extra cash while she paid down $53,000 in student loans.