Reimbursements shops are a really gray area, and you are unlikely to get consistent answers here. I personally don't claim mileage on shops for reimbursement only, because I don't make any income to deduct from.
My CPA told me that in addition to the milleage office supplies l I am able to take my phone, and internet charges since I use both of them a great deal for shopping. I really use it more than ' he reports. t
Mystery shopping wouldn’t be employee expenses, it would be deductible on schedule A as hobby expenses, if you report your income on form 1040, line 21 as hobby income.
Since there are so many different factors involved, there really is no “best” way to do it, it isn’t one-size-fits-all.