The post office can be used to send mail to villagers in your town as well as downloading free items from Nintendo. Speak to Pelly or Phyllis at the desk to do this.
If you go to the ATM you can deposit money into your account and pay off your house loan. On the first of every month you get 0.5% interest on whatever is in your account.
You also get a letter from the Post Office once you’ve saved a certain amount which includes a prize:
|100000||Box of Tissues|
|10000000||Post Office Poster|