In some cases you also get to keep the items you have bought while doing a Mystery Shop!
You can do a Mystery Shop for a Gas Company and get FREE Gas.
You then use that FREE Gas to drive to a Restaurant to have a FREE Lunch, then go to a different Restaurant to have Free Desert and Coffee. After you have recouperated from your hard morning you can go shopping for Clothing, Shoes, Cell Phones, and many other things and know that each time someone is going to pay you to shop. Such a hard life.

If you like to travel a bit you can also get shops for Hotels which re-imburse you for the room cost, re-imburse you for the wonderful meal and drinks plus pay you on top of it just to report what you see while there. If you plan it right you can get paid for most of the expenses of a Wonderful Weekend away from home.