I have always done my own taxes. I keep my records as a spreadsheet that shows the detail of both the company and the client, the amount of the fee and the amount of the reimbursement and any unreimbursed expenses. If I were going to a public preparer I would make a copy of my spreadsheet without the client name. I would not take the receipts to the public preparer.
Purchases you make are taxable to the same extent for you as a shopper as they are to any normal customer. In my state, food from grocery stores is generally not taxable, but food in a restaurant is taxable. But reimbursements are not taxable to you as income.
Example: I do a sandwich shop with specific requirements for a $10 fee with a $10 reimbursement. The required purchase is $11 and we have 7% sales tax so I spend $11.77. The company sends me $20 for the job. Fee $10 - taxable; reimbursement $10 - non-taxable; unreimbursed business expense $1.77 - a deductible expense.
Unless it is specified in your instructions to the contrary, what you purchase to satisfy shop requirements is yours to keep. You are free to keep it, give it away, throw it away or resell it if you wish, but you are not to return it to the store for your money back.