It's been quite a while for me, but I've done a number of these, albeit from a different MSC. The ones I did carried no reimbursement, but were strictly an audit.
As I recall, the problems were that they were picky, picky, picky about the photos. (Don't know if it was the MSC or the Client.) The stores were quite small with narrow aisles and it was difficult to get full frame photos, particularly the checkouts showing front and wall from top to bottom. The report was somewhat time consumming in matching up the photos with the proper questions.
And finally, since they were mostly franchises, the store owners were usually quite defensive as they felt the Client was doing this to force them into stocking merchandise that didn't sell.
Aside from that, they wweren't too bad. If the fee is around $20, it is worth it.
Good luck.