This is a scam. Since it was mailed to you, Report it to the US Postal Inspectors. You may want to report this to the police. The scammers are getting good at forging checks and using emails that look almost like the real ones. And they generally use the name of real mystery shopping companies. Using Priority Mail helps to convince the poor souls who accept the bogus job that it is real. Trust me, even if only a small percentage of people agree, the scammers make a lot of money.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008