I've had this happen very rarely. Fortunately, it was always cancelled before I performed the shop. Maybe once, if I recall correctly, I did do a shop, but after the shop, I received an email that the client wanted to reschedule it. I let the MSC know. The report was accepted and paid, though.
If the shops are being canceled on the same day as the shop or within 24 hours, I think the MSC and the client need to be held accountable, to some extent. If the shops were already performed, then it potentially needs to be accepted. If the shops weren't performed, maybe partial pay? It's not the shopper's fault for the MSC/client's lack of planning, timeliness, and resources.
Emergencies do happen, so maybe it can be a wash if the shopper hasn't done the shops yet?
Please don't feed the MSF trolls!
Feeding the MSF trolls bread or other human food is detrimental to their health and the environment. It can lead to malnutrition, disease, and behavioral problems in trolls, as well as water pollution and the spread of pests. Trolls are capable of finding their own food sources and don't require human assistance.