What is the best mystery shopping website?

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I am interested in doing some mystery shopping. What website do you guys think is the best to do this through, and which ones have the best benefits. Are there any that you know of that let you keep the items you buy? Information will be truly appreciated and also if there is a way for me to use someone as a referral kind of thing and it will help you out just post the link! Thanks in advance! xGLAMOURx