1. From within your TaxAct return (Online or Desktop) click on the Federal tab. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
2. Click Itemized or Standard Deductions to expand the category, and then click Unreimbursed employee expenses - Subject to 2% of AGI limit
3. The program will proceed with the interview questions to enter the appropriate information 
   

Per IRS Publication 463 Travel, Entertainment, Gift, and Car Expenses, page 15:

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 reimbursement amount by more than the rate of inflation.