I would think the circumstances would come into play if you were audited. For example, the Sonic shop I do gives me a choice of certain things we are allowed to get. The one I choose ends up about 50 cents over the reimbursement. I claim the full amount I paid, because there is no reasonable way to be sure to stay within reimbursement due to variables such as price increases and taxes and the need for confidentiality that would stop us from saying "Wait, I can't go over $5.80, let me pick something else" when they tell you the total.
But if the job requires you to make a purchase, and you know you are only going to be reimbursed $4.75, and could have stayed under $4.75 but chose to go over for personal preference reasons, and you knowingly go over because you want a more expensive item why would the IRS allow deduction of the full price? That's not reasonable at all. (I'm not talking about ending up $5.25 with the tax, I'm talking about buying a $10 bottle because you prefer that brand, or wanted a bigger quantity.)
I did an Ace Hardware shop, $5 reimbursement, and bought a $20 bottle of weed killer I needed. I deducted $5 for the "required purchase" but I sure wouldn't think of deducting the other $15 off my taxes. Even when I spent $6 because I bought the required item for $3 and added an impulse item at the register, they reimbursed me $5 but I wouldn't think of claiming the other dollar as a deduction. If it wasn't necessary to go over, then it's not a deduction. (I was surprised they gave me the $5 instead of just reimbursing the "required" item.)
In other scenarios, you are required to make a purchase for a $2 reimbursement but there is nothing you can find for under $2 so you make a $4 purchase. Certainly deduct the whole $4. You were required to make a purchase and couldn't make one within the reimbursement. The $4 became a necessary purchase.
The best thing to do, unless you were FORCED to go over because there was nothing under the limit or the designated purchase (pick one of these entrees) put you over, is to just deduct the $4.75 you were allowed, and consider it a discount on something you wanted to buy anyway. After deducting the full allowable mileage deduction, you're not likely to make a taxable income mystery shopping unless you do this full-time. Don't inflate your losses needlessly by claiming expenses you didn't have to make.
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I pray it does not occur that the last thing I did before I died was vacuum the house or eat broccoli.