MSPA issues certificates, gold or silver. I can't tell you much detail as I'm not certified, nor have I worked for a company who requires certification. Some of the companies will ask if you're certified, some may say they prefer if you're certified. From what I've read in this forum, and others, certification costs money, and doesn't necessarily gain shoppers more work. It seems to be more about bragging rights. Maybe certification would give you an edge if you live in an area where there are tons of shoppers competing for work - or maybe not.
I haven't paid anything to be a shopper, and have done just fine.