Assuming that you are in Japan now. Imagine the following the case.
You have things to send cash to someone.
You went to buy a convenience store ano by some envelops like this.
You put cash into the envelop
seal it, and then you put the same value of stamps on the surface of the envelop, as the other way of sending documents.
Is it right way to send cash?
The answer is NO. That’s illegal to send cash by normal post.
You should send cash by way of “Genkin Kakitome”, registered mail as follows.