Basically they sell two levels of certifications and it is questionable whether either reflects your ability or teaches you anything you did not know from childhood. Many, but not all, legitimate MSCs (Mystery Shopping Companies) are members of the MSPA, many legitimate MSCs are not. Being a MSPA member does not guarantee that an MSC treats shoppers well or even appropriately. The MSPA is an organization for the MSCs, not an organization to protect shoppers. The sale of certifications merely gives the MSPA additional revenue with which to work.
Some shoppers indicate there have been benefits from having purchased certifications, some have indicated it has made no difference in the type, quality or numbers of shops available to them. It is certainly no guarantee of anything except that your pocket will be emptier for having spent money on certifications.