health insurance card

I am an exchange student at Shinjuku, Tokyo. I left Japan this month. Before leaving, I went to Shinjuku City Ward Office to cancel my national health insurance. I went through all the procedure. At last, the person in charge gave my an envelope of health insurance card. He told me to post it at the airport. However, I left in a hurry so that I forget to post it. Now the card is still in my control.I wonder have I successfully cancel my national health insurance. Should I do anything more for it?

He told me to post it at the airport. However, I left in a hurry so that I forget to post it.
So just post it back where you are now?

Most probably, ward office set the cancellation date right at the last date of your departure and let you keep the card until then, in case something happen and you might need it.

Did you made any payment, the remaining payment of your coverage?

yes, I did the remaining payment and filled all the documents. So it maybe like what you said. They let me take it in case that accident happens. Thank you very much