If you're away from home overnight, you may do better to take the per diem rate for meals and/or hotel. If you travel more than 50 miles from your town, you might be able to take the per diem rate even if you don't stay overnight.
I would be careful deducting more than the minimum required to do the shop. Usually they tell you to buy some token item just to get a receipt (such as a pack of gum). I wouldn't try to deduct a $5 gas purchase that was not required for the shop.
The controversy about the "mileage to the first shop and from last shop to home" being deductible or not arose because the IRS instructions state that you can deduct mileage from a "qualified home office" to a work stop. in other places they specifically state that travel to your first work location of the day and the trip from the last work location to home is *not* deductible; it is considered commuting mileage -- even for someone who is self-employed.
So we need to determine if the home "counts" as the "first work location" which would make the round trip entirely deductible.
The problem is that "qualified home office" has a specific definition for the "home office deduction." It does not have a specific definition in relation to the "mileage deduction." In order to take the home office deduction, there must be a space in the home used "regularly and exclusively" for the business in question.
The question, which is *not* addressed in the IRS regs is whether a "home office" exists if it is not claimed on the tax return. Some people believe that the mere fact that we do our shop-hunting and reporting and recordkeeping in our home automatically makes it a "qualified" home office even if we *do not* take the home office deduction on the tax return. The situation is simply not addressed in the regs.
My feeling is that the word "qualified" is a signal that the "home office" must meet certain requirements. Since the only place the requirements for a home office are defined is in relation to taking the home office deduction, I believe (and was told so in a small business seminar I took when I first became self-employed) that I can only deduct mileage to the first stop *if* I actually claim the home office deduction.
Others believe (but I cannot confirm this specifically in the regs) that merely being the principal place of business qualifies for the "first and last leg" mileage deduction.
I suspect in the real world it comes down to who you get for an auditor.
My suggestion (as a tax preparer) is to avoid the issue by simply having a designated space in the home where you keep some of your mystery shop paraphernalia and regularly sit there to do something related to mystery shopping -- and don't let your kids use the desk for homework -- and claim it as a home office deduction. It can be nothing more than a desk where you set your laptop when you do your mystery shopping work; the computer wouldn't have to live there permanently. It just needs to be a designated space, it does not have to be an entire room. I claim a 20 square foot "office" for mystery shopping -- a desk where I store my shop documents and travel laptop and sit down to do my reports on it -- and that means I don't have to worry or wonder if "my" auditor will interpret the regs strictly or logically. Having that "office" on my tax return legitimizes my mileage from door to door.
I have a different home office I use for my accounting business. Different desk, chair, computer, printer, scanner, and file cabinets. So whether I am traveling on a shop or traveling to pick up and deliver tax returns, the first and last leg of the trip is deductible.
But you and your tax preparer will have to defend any audit on this point so the best thing to do is to talk to them about it and follow their advice. Auditors work regionally; some parts of the country may simply take a different approach to this question. All I know is I cannot find a reg that specifically addresses the issue of taking the mileage when the departure point is not a qualified home office that was taken as a deduction on the tax return. I haven't been through an audit where this was a factor, and even if I had, there is no way of knowing if whatever decision was made would apply to the next auditor.
Time to build a bigger bridge.