Here’s more about the service they use, Surface Air Lift (or Lifted), SAL: https://www.post.japanpost.jp/int/service/dispatch/sal_en.ht... , https://en.wikipedia.org/wiki/Surface_Air_Lifted

I don’t know the details for Japan, but almost every country‘s post has been affected: https://www.linns.com/news/us-stamps-postal-history/internat...