I do some mystery shopping with a company called Catalyst based in Dublin. They have a number of high profile cutomers, and i've visited and phoned a number of different companies over the past year to report on customer service, or whether they are promoting what the company wants them to promote.
I've been asked to purchase various products but do get re-imbursed for it, normally its a max spend of approx Eu15, and they provide you with a number of possible items. You get paid Eu20-30 on average for the shop, and you are responsible for advising revenue of this additional income, the comany do ask you to sign a sheet.
As another poster has said it can be very detailed in terms of the information you need to pick up, names / descriptions / availability of products / what selling staff try and do / state of the store.
Having worked for a company that did get mystery shopping reports, and part of our bonus was based on our cummulative scores, I'm happy enough that its a valid form of checking customer service.