The cutest and smallest post office I have ever seen in Japan is located in the town of Fujikawa in Yamanashi Prefecture. The name of the post office is Ohkubo Simplified Post Office: https://map.japanpost.jp/p/search/dtl/300108865000/.
Ohkubo Simplified Post Office, Fujikawa, Yamanashi, 12/22/2024
There is a smaller post office than this one in the United States of America. Ochopee Post Office is considered to be the smallest post office in the USA, located in Ochopee, Florida. How small is it? You will find it: https://facts.usps.com/smallest-post-office/; https://en.wikipedia.org/wiki/Ochopee_Post_Office#:~:text=Measuring%20a%20mere%2061%20square,41%20and%20State%20Road%2029.
The Smallest Post Office in Japan is Hikutani Simplified Post Office in the city of Yamaguchi in Yamaguchi Prefecture; however, the service of this post office has been suspended since October, 2015 until someone raises a hand to help. Currently, there is only a post box at the site of the Post Office.