Rasky brings up another point, which is that shops with no fee start to approach the issue of it being an in-kind payment, and then technically taxable, so I think the tax burden could potentially be set at $15 of income at that point, considering a $10 deduction for mileage.
Shop structures like Goodwin offers that give you a flat fee and have a required a purchase will definitely require reporting of 100% of the income, and then deduction of the expenses.
As far as professions that first the Pub 463 criteria can apply toward:
-A babysitter takes kids out for ice cream, gets reimbursed.
-A PI watches a cheating husband in a bar while having a meal, gets reimbursed
-A food critic or travel writer researching 'the best taco in town', gets reimbursed
-etc.
...and agreed you can infer that allowable deductions must then be reported, but I believe their must be another rule that applies more closely to the mystery shopping model (and that pub 463 is not designed with MSing in mind), because if you read that publication further, it says that the client who reimburses the contractor must then be subject to the 50% allowance on the meals, and I know for a fact the MSCs do not shoulder the tax burden for 50% of the meals that shoppers eat.
Hotel reversals, in essence, are a refund. They don't forgive the debt. the charge remains on your credit card. A refund for the exact value paid is simply applied to the card. For cruises, you are simply issued tickets without having to purchase them in most cases. If a ticket purchase is required, it is then reimbursed.
The argument for the reimbursed items not being in-kind benefits (as explained to me by an accountant for a MSC I used to work for) is that the shopper does not receive the full benefit of the experience. They have to be working throughout the time there and have severe limitations placed on what can be ordered, who can come along, etc.. The example of a taxable in-kind benefit or barter would be more like a hotel giving a shopper 2 free nights + all expenses in exchange for a report about the efficiency of the staff.