1. You can't deduct non-reimbursed employee expenses after the tax reform act. Such expenses may be deductible on certain state tax returns only, but not your federal return.
2. You can deduct out of pocket expenses you pay for in the course of providing service to a charity. Whether you get any tax benefit from buying office supplies and donating them to the charity will depend on your overall itemized deductions. You need a letter from the charity confirming that you donated items for the charity's use, although the letter does not have to attest to the value (you prove that with receipts).
3. The employer can reimburse you tax-free under an accountable plan. You must give them receipts showing that the expense was business-related and proving the amount. They would then list the reimbursed expense as a routine business expense on their books.
4. If you are in a position of financial authority, it would be best if you got someone else to sign the reimbursement check, or got someone else to sign the donation acknowledgment letter, so it does not look like self-dealing.