No post office at Kansai or Fukuoka — but there is a counter open 7am–9pm daily. Where to collect pocket WiFi and SIM cards...
WiFi Return Postbox Locations at Major Japanese Airports
Returning rented WiFi at Japanese airports? Find WiFi return postbox locations at Narita, Haneda, Kansai, Fukuoka, New Chitose, Chubu, and Naha airports here....