On a Payless shop it would be possible to buy something for less than the allowed reimbursement: shoestrings, shoe polish, perhaps a pair of socks, etc. Thus what you choose to spend beyond the reimbursement is not an unreimbursed expense but a personal expense.
On the other hand, I did a Leslie's Pool Supply a few years ago with I think it was a $1 reimbursement. Their indication was that there would be small items available for $1 or less. There were not. The cheapest thing I could find was an awful little cheesy 'tool kit' for $4.99. I chose not to purchase that but spent about $10 on a gag gift for a pool owner. I claimed as 'unreimbursed' $4.29 and 'reimbursed' $1. Here was the logic: If I bought the cheapest thing in the store and paid tax I would spend $5.29. I was reimbursed $1, which means I would have been out of pocket $4.29. I spent more than that but felt I was entitled to claim only what I would have been out of pocket if I had purchased the cheapest item. I have not been through an IRS audit, but I would be perfectly comfortable explaining why I claimed what I claimed.