Per IRS Instructions for Form 2106 Employee Business Expenses, page 15:
Line 1. If you were a rural mail carrier, you can treat the amount of qualified reimbursement you received as the amount of your allowable expense. Because the qualified reimbursement is treated as paid under an accountable plan, your employer shouldn't include the amount of reimbursement in your income.
You were a rural mail carrier if you were an employee of the United States Postal Service (USPS) who performed services involving the collection and delivery of mail on a rural route.
Qualified reimbursements. These are the amounts paid by the USPS as an equipment maintenance allowance under a collective bargaining agreement between the USPS and the National Rural Letter Carriers' Association, but only if such amounts don't exceed the amount that would have been paid under the 1991 collective bargaining agreement (adjusted for changes in the Consumer Price Index since 1991).
CAUTION! If you were a rural mail carrier, do not use Form 2106. Your employer should not include the amount of reimbursement in your income.