The Shopper is paid for their time, and usually compensated for some amount of purchase. This is to ensure they can blend in with your regular customers. The Shopper sends in the receipt and very specific details, so there is never doubt that the Shop was done as described.
A Customer is the most important visitor on our premises.
He is not dependent on us. We are dependent on him.
He is not an interruption on our work. He is the purpose of it.
He is not an outsider on our business. He is a part of it.
We are not doing him a favour by serving him.
He is doing us a favour by giving us an opportunity to do so.
"Mahatma Gandhi"