I guess I am lost of being.mystery shopper...

If the Mystery Shopping company you are with is a genuine one, your task was probably to see and report on the shop's reaction to being asked to provide three things on a limited budget. Mystery shopping is a way in which shops can gain reliable information about customer experience. Sometimes the tasks they set are ones which are bound to fail (because you could not possibly buy three things for $15 from a shop where the minimum item price is $10). What they are very likely looking for is your experience of how the staff treated you as a customer and whether they were helpful, indifferent or scornful of you. A successful mystery shopper should understand what the task is about, be able to perform it in the role they are asked to play, be observant and able to write a good, detailed and accurate report very quickly. The pay for such high quality work is actually rather low, though many people are attracted to the job because of the "perks" (of 'free' goods) which they can supposedly acquire. If this was your first assignment, it was probably intended to be a test of your reporting ability more than anything else.