@Susan L.
How is the MSP/MSC going to pay full price if the client isn't going to cover that cost? It's a crap situation. I would argue as a shopper, it's my responsibility to check that what I'm about to go and do is going to be there like expected. Yes, the MSP/MSC should give the correct information, but they are not compiling the data, but instead setting up the shops on whatever platform they use based on what the client has instructed them.
This is why I generally do not pick up new assignments or random things. While in theory, some may be easy to complete, I don't like trying to learn new rules/scenarios for a limited time offering. If this flu shot shop was going to be around monthly, then I would change my mind, but the subject of "flu shots" has me believing this is a short-term project that at $15 is not worthy of my time to learn and mess with. But I don't do this for a living either, so I can be a little more selective with that I take.
My point is, I don't set out to do shops that I am uncertain if the business is there or not. I am only taking shops for places I already know. And If I don't know, I'm checking google maps or some other tool to familiarize myself before heading out the door.