"Secret shopper" job? Is this a scam?

I’m trying to determine if something is a scam. Today, I received a letter from a consumer research company in Toronto asking me to be a “secret shopper.”

They included a check for a sizable amount of money payable to me with instructions to spend a specific amount at specific big box retailers (I get to keep the products), and then fill out a survey and fax it back to them.

After the purchases, there will be $250 left over as payment to me.

The letter is very professionally written. With company logos and professionally typeset. It looks like it really could have come from a legitimate market research company. And the terms seem reasonable. Nothing too good to be true.

It lists various company reps at the bottom with phone numbers, but calling them won’t prove anything, will it?

Yet, I can’t help feeling that this is almost certainly a scam.

Has anyone else received this?

If I deposit the check and wait a week for it to clear (and then wait another week or two) before I spend any money, will I be safe?

I’d rather not state the name of the company or the retailers here in case they google themselves and find this question on Fluther. PM me if you want all the gory details.