This would be reported as an employee job expense and reported on your schedule A if applicable.
According to the IRS:
"Rural mail carriers. If you are a rural mail carrier,
you may be able to treat the qualified reimbursement you received as your
allowable expense. Because the qualified reimbursement is treated as paid under
an accountable plan, your employer shouldn’t include the reimbursement in your
income.
If your vehicle expenses are more than the amount of your
reimbursement, you can deduct the unreimbursed expenses as an itemized deduction
on Schedule A (Form 1040). You must complete Form 2106 and attach it to your
Form 1040, U.S. Individual Income Tax Return.
A “qualified reimbursement” is the reimbursement you receive
that meets both of the following conditions.
It is given as an equipment maintenance allowance (EMA) to
employees of the U.S. Postal Service.
It is at the rate contained in the 1991 collective bargaining
agreement. Any later agreement can’t increase the qualified re-imbursement
amount by more than the rate of inflation.
See your employer for information on your reimbursement.
If you are a rural mail carrier and received a qualified
reimbursement, you can’t use the standard mileage rate."
Report the expense under job W2 expenses.
You can also type 2106 into the search box and click on the "Jump to 2106" link.
