That evening the Post office called my home to let me know that they had my change and they would keep it for me until I picked it up.
How did they find me? From the return address on the package I had sent! Today (almost a week after sending the letter) I went back to that Post office and retrieved my change.
All for 38 yen.
I continue to be humbled by the service levels in Japan, and from the Post Office no less.