I should be "certified" for being a mystery shopper, but not by the MSPA
An "orphan shop" is a single shop out somewhere where you don't/didn't plan to go or need to go anyway. Because any/all mileage and time necessary to do the shop belongs solely to that shop, for it to be profitable the shop would need to pay more than it would if it was part of a string or route of shops you were doing on a particular trip.
Orphan shops are generally not the scheduler's problem unless they are asking on short notice for you to drop everything and GO. My gas station 60+ miles away on Tuesday was arranged and scheduled such that I had about 15 minutes to get out of the house and on my way to do it if I was going to beat the deadline. On that short notice there is no time to even review the instructions meaningfully, much less find other shops and read instructions for them meaningfully before you need to be on your way. And obviously you wouldn't schedule jobs out that way in hopes that you would be offered a last minute shop. So a scheduler has to realize that it is not just greed that makes you name an outrageous sounding price, it is a business reality.
And on IRS and mileage. There are two ways you can do it but you must be consistent year after year. The first is to actually work out the annual depreciation of the vehicle, keep track of costs of fuel and maintenance and repairs and licensing and insurance and all the whoopla. Then you figure what percentage of use of your vehicle was personal and what percentage was for business. You can take the business percentage of your overall costs. (To figure the business versus personal mileage you must keep tabs of mileage for the full year and mileage for business, and you need to do this for either method you choose.)
The second way is mileage, where you simply keep track of the business miles travelled. For those of us who don't have a job other than mystery shopping from our home base, this is straightforward. From backing out of the driveway until we turn off the ignition when we return home. For those who do shops on their way to or from working another job, they must figure their "commuting miles" are not deductible and need to read that information in the IRS bulletins closely.
For the 2007 tax year, business mileage was figured at 48.5 cents per mile. For 2008 it is 50.5 cents per mile. If gas keeps going up in price I'm sure the 2009 deduction will be even higher.
IRS also allows you to deduct tolls and parking for your jobs. I see bridge tolls in my area and parking meters ranging from 5 cents to 25 cents for the fifteen or twenty minutes needed to do most shops. This is in addition to the mileage expense.
I prefer the mileage because it is simpler, my vehicle rarely needs repair, my routine maintenance is mostly done by shops, some of my gas is done by shops, etc. I would personally be uncomfortable claiming the $39 oil change reimbursement as a business expense and then turning around and calling it also a vehicle expense if I was using the percentage of personal/business actual expenses method. Similarly since my vehicle is almost 10 years old there would be little depreciable value left to it. But even with a brand new vehicle, after a couple of years the depreciable value would drop such that mileage would be the better way to claim the deduction but you would have been "locked" in to the actual expense method if you had chosen that.