A mystery shopping project starts with an email invitation to fill in a brief screening questionnaire. This is to check whether I am part of a typical customer group of the company to be tested – I don't want to stand out by looking out of place in the shop!
If I am suitable for the project and are assigned to the job, I will get a full briefing. This will usually include written documentation and an interview with the project managers. That way, both parties can be sure that everything has been understood and I can happily proceed to the assignment. Needless to say I will be careful on location and ensure nobody notices that I am a test customer!
When I have completed the assignment, I will fill in a questionnaire about my experience on a tablet PC or at home. I will know the questions in advance so I will be able to make sure during the assignment that I can answer all the questions.
By the way, I rarely buy anything on test-buying assignments. It is frequently just the quality of the advice that is tested. If I have to buy something, for example to test the payment procedures, I will be told exactly in advance how to cancel the transaction or get a refund.