There are a bunch of different "certifications" in this industry. When a company has one for their jobs it is the specific training and information they want for that specific type of job. So usually before you can accept an Elephant National Bank, you will need to go through their training and quiz. Some places make the training AFTER you have accepted the job, but most are before. Certification with them for Elephant National Bank may last just for the current round of jobs, may last for a month or may last until there is a change to the requirements. These are no cost.
In only one case I can think of (Corporate Research) you need to take the training before you can see ANY details about the job including the fees.
Then there are the pay-to-shop folks who offer certifications that nobody wants, nobody uses but you pay for. NAMS is apparently doing that as is Shadow Shopper.
Then there are the Silver and Gold certifications offered by the MSPA (Mystery Shopping Providers Association) which is a trade association of the companies that hire us. They are $15 and about $100 respectively. There is plenty of debate on this and most other forums about whether they are worth a dime. They are at least presumptively recognized by some member companies of the MSPA but not all of them. I can think of a whole lot of better ways to spend your money. If you are somewhat bright and reliable, you will do very well without them.