Mystery Shop Providers Association is an easy organization to join. Membership used to be free but now it's $20 but there are many discounts to popular retailers as well as job leads. If a person lives in a large city, this could be decent part-time income. I got my Office Depot card years ago and it's been at least a year since accepting an assignment but I can still get thing like photocopies, stationery and some electronics for up to 40% off.
It is interesting to know being a mystery shopper gives you a discount on some purchases. I would like to do a mystery shop offer but the fact that they ask me to spend my own money and wait up to 6 weeks to get paid is a problem for me. I saw one today that is a taste test, the pay is $10 in 30 days and you have to make a small purchase.