I prefer to think of MSCs as clients, for whom we perform a service. "Customer" sounds to me more like a person or an entity that purchases a product. When I sold real estate, it was drummed into us that the seller whose listing you had is a client, not a customer. And the person who purchases your listing is a customer. Unless you're representing a buyer in a formal buyer agent relationship; then he/she/them is/are your client.
It might sound like an irrelevant distinction, or no distinction at all. But I don't think so. In my freelance communications business, the companies for which I work are my clients, not my customers. I'm not selling a commodity, but am providing a professional service. (But in my online business, the people who buy physical merchandise are my customers.) Same with mystery shopping. Maybe if shoppers saw themselves as professionals who provide services to clients, they would value their own time more. Obviously, there are many, many shoppers who do have this mindset; I'm not suggesting otherwise. But new shoppers, in particular, might not place the value on their services as highly as they should. A simple tweak in terminology might make a difference. I know it might sound silly, but the words we use can alter perspective, even if just a bit.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.