Hello! And welcome to the Forum. If you are thinking about getting into mystery shopping, please look at the topics in the link below...it is from the New Mystery Shoppers section here on the Forum. Many of your questions will be answered. [www.mysteryshopforum.com]
As to your main question here, if you receive ANY sort of large check that you were not expecting, and told to cash the check, keep so much and send the rest back as gift cards, or something similar, then THAT IS A SCAM!!! Do NOT fall for it. If you receive any large check that you did not do any work for AHEAD OF TIME, then that is a scam as well. Do NOT cash/deposit these, because you will have to pay it all back! Hope that helps.