Its a scam, turn this over to postal authorities or email provider, don't do it or you will be out $2000. Legit MS companies do not operate this way, might want to advise Trendsource also as they are a legit firm.
TrendSource does not work this way. The person(s) who sent you the big cashier check is not TrendSource. They are using TrendSource's name to scam you. This is not even an unusual scam. Let me guess. You are to deposit the cashier check in your personal account and get cash. After you buy the credit cards, you call them and they empty the cards immediately, taking the $2,000. Then - surprise! - your bank will reject the cashier's check because it will be fake and they'll charge it back to your account. You will be out the $2,000 you spent for the credit cards, you won't get a fee, and your bank may close your account for fraud.
Beware if you receive a large cashiers check, personal check, or postal money order from someone who wants you to buy credits cards, gift cards, itunes, etc. Beware especially if you are contacted by a mystery shopping company who you did not already sign up with. That's a clue. If a company does not know who you are, why would they work with you? If a company has not collected your personal information and your SS#, they cannot legally pay you. Another clue.
I don't know of ANY legitimate mystery shopping company that pays you BEFORE you do the job. That's just not how it works. Except for Nevada, you are lucky if they pay you within a month.