This is totally and completely a scam. Do NOT cash the check. Do NOT buy the cards.
There is a real company called "Sinclair," however it is very common for scammers to use real MSC names to do their dirty work. Real mystery shopping companies will pay you several weeks *after* you do the work. Purchases will be made and established retailers, as will returns.
Here's how the scam works: You deposit the check. It looks "okay" and the bank gives you access to your funds. You then buy the gift cards and mail them, never to be seen again. About two weeks later, the bank realizes that the check is fraudulent and withdraws the money from your account, charging you any fees in accordance with the details of your account, such as overdrafts.
Hard work builds character and homework is good for your soul.