From Nookipedia, the Animal Crossing wiki
The Post Office is a building in Animal Crossing and Animal Crossing: New Leaf where players can mail letters to other players or to animals living in the town. In Animal Crossing: Wild World and Animal Crossing: City Folk, the post office is merged into the Town Hall, but in Animal Crossing: New Leaf, the post office is again a separate building.
In Animal Crossing, The post office is always located to the north of town. In addition to depositing mail for delivery, the player may also pay off their loan, store money in a bank account, and also save old letters, which can be used for additional storage of items. On the left side of the office, there is a small machine that lets the player enter E-Reader codes. Directly outside the post office is a billboard where the town tune is displayed, which can also be edited inside.
During the day, the clerk inside will be Pelly, who is polite to the player; during the night, however, the clerk will be Phyllis, who, unlike her sister, is rude. In addition, Pete, who serves as the town's postman, also works here.
In Animal Crossing: New Leaf, the post office returns as an independent building, separate from the Town Hall. Once again, the post office serves as the place to mail letters, and it now includes the ability to receive items from Nintendo via SpotPass. The player's debt to Tom Nook can be paid at the ABD located in the Post Office. Similar to its appearance in Animal Crossing, Pelly will serve as the post office's clerk during the day, while Phyllis acts as the office's clerk during the night. Additionally, Pete continues his role as the town's postman, delivering mail.