I am 99.999% sure it is a scam. There is an ATH POWER CONSULTING CORPORATION that has "Apex" on their web site but trust me, it is tough to get $15-17 out of them for a job, so sending you $900 to keep $200 sounds like a true scam. Don't cash the money gram.
You will not be sent money 'out of the blue' by any legitimate mystery shopping company.
For information about mystery shopping, I would suggest you start with the "Nuts and Bolts" thread of this area of the forum and the other "Sticky" threads at the top of this section of the forum.