Mercantile is a real company but this is a pretender using the Mercantile name to scam you.
Ask yourself: why would a legitimate company send a check to a stranger before any work is performed? In addition, without collecting your SS# or EIN, how could they report your income? They don't want your SS or EIN because they are not paying you, they are scamming you.
Mercantile is a legitimate company but they do NOT send checks in the mail to someone who has not already performed work. SCAM. The check they sent you will bounce and your bank will want the whole amount back - from you. To add insult to injury, I've heard many times the bank closes the victim's account.
Common sense should tell you that it's a definite scam. Besides all the other red flags, simply the amount of money they are paying should be a tip-off. Most mystery shops pay $5-$30. Why would anyone pay a stranger a large fee for a job which requires no skill, training, or expertise - how much pay per hour does the fee they sent you work out to? And what is the minimum wage? "Oh, my! I can work at Walmart for $11 an hour or I can get a big check in the mail and cash it and perform an easy job." Hmmmmmm