You shouldn't feel any obligation to tip your mail carrier on a day-by-day basis, but it's appropriate to reward the carrier's year-long work during the holiday season. The United States Postal Service prohibits its carriers from accepting cash or check gifts and gift cards, which means you'll have to be a little more creative in your tipping approach.
Keep Gifts Under $20
The U.S. Postal Service allows you to give gifts to your carrier, provided the gift meets specific criteria. The value of a gift shouldn't exceed $20. Acceptable examples include perishable items such as snacks or drinks, or convenience items such as hand warmers or a travel mug. As an alternative, consider giving your mail carrier a letter or holiday card offering thanks for the carrier's exemplary service. The carrier might appreciate a letter of thanks sent to the office supervisor.