I think perhaps you have found "Mystery Shopping Providers Association", referred to here more often as MSPA It is the trade association of the companies that hire shoppers as independent contractors to do mystery shopping. They do not offer shops themselves. More than likely if there is a Mystery Shopping Providers that wants you to register as a shopper to work for them it is a scam.
The real Mystery Shopper Providers Association may be asking that you as a shopper register with them and perhaps will try to sell you opportunities to buy 'certifications'. You do not need these to be able to shop.