Secret shoppers, advice please?

I have done mystery/phantom shopping. I was called to do specific ignments. First I had to go to the office to get the instruction, money and doents to fill in. Then at the given time I had to enter the target shop, buy an item and observe the shop istant. Then I filled in the questioner and evaluated lots of things about the shop and the istant. Then I went back to the office, gave back the item, the change and the questioner. What I did was just an occasional thing. When I wanted to quit (I found a full-time job) I just told them I am no available any more.