I agree that mystery shopping could have lots of gray areas, depending on the point of view. It's conducted and reported covertly and is full of mystery

My better half, for instance, has a problem with churches being shopped. As with accepting fees, accepting shop types is to each his own.
If not prohibited, reimbursements, as fees, are mine to do with as I choose. Otherwise, the MSP doesn't care if I keep, giveaway, or return the thingamajig.