Lets take this back a step or two. Are you signed up with legitimate mystery shopping companies? There is a list here on this forum and there are lists at Volition.com and MSFreedom.org just to name a few forums with lists. I don't ever remember these legitimate companies talking about "shopping for free".
With a legitimate company you self-assign or request a specific job for a specific location that they have listed on their company website (where you signed up). When that shop is assigned to you, you will go to that location during the time frame they list (which may be between a few days, or may be a specific day, may be between certain hours of a few days or a specific day). If the job is "Fee only" you will be paid the fee that they have posted for the job. If it is "Fee plus reimbursement" you will most generally see a dollar amount as a maximum reimbursement for something that you purchase and submit the receipt for. Sometimes your purchase can be any item of your choice--most usual when the reimbursement is only $1-$2 and they are looking for the receipt to prove you were at the location at a certain time on a certain day.
Sometimes reimbursements are for required purchases that the company specifies you must buy. This may be $5 worth of gas at the pump and up to $1 inside purchase. If they specify like that, you must buy AT LEAST $5 in gas at the pump and some purchase inside the store. If you buy $4 worth of gas your shop will be invalid, though you can spend MORE than $5 at the gas pump if you want--but you will only be reimbursed for $5 in gas. If you buy at least $5 in gas at the pump and spend $.79 inside you will be reimbursed $5.79. If you buy $20 in gas and spend $1.59 inside you will only be reimbursed $5 for the gas and $1 for the inside purchase.
99.999% of the time you are putting your own money up front to make the required purchases and should you mess up the shop by not following instructions you will not be paid OR reimbursed.