Through the years, I have discussed the issue of flaking with the owners of two small MSCs and a few schedulers. Realizing the numbers would be based upon the individual assignment, the company's pay and the experience of the shopper, the numbers I recall ran from 10 to 50%.
16 years ago, a security company, Graymark, decided to enter shopping. They folded that branch after four years, with the head stating to me the aggravation of flaking was a reason. What was strange to me was they paid well and were quite easy to please. I received $15 + an open ended reimbursement, a week to complete the shop, no guest or ordering requirements and, until they were being screwed, no receipt necessary.