In this business there is the client (who ordered the job), the company (who puts together the job and reports back to the client) and the scheduler (who finds someone to do the job). Some clients do their own scheduling and shopping, though they are overall infrequently found with mystery shopping. Some companies do both the job work and their own scheduling, and these are the ones I most frequently work with. Some companies are small enough or have small enough databases of shopper on their own, that they use outside schedulers. Palm, DSS, Kern and others are the outside scheduling services. In some cases it would appear that 'outside schedulers' also have some clients of their own, though this appears to be relatively rare.
My personal preference is NOT to work through outside schedulers because there are too many layers needing to be paid and too much ambiguity as to with whom your contract is and with whom to contact regarding payment. If I go to a company website and sign up for a job, my assumption is that I am contracting WITH THAT COMPANY, not with some scheduler. If a scheduler is handling the work, so be it, my contract is obviously with the COMPANY since that was the location where I agreed to do the work.