1.
registered mail for sending cash
The postal service for mailing cash: the money goes in a special envelope sold at the post office, is sent by registered mail with a record of each step, and is compensated up to the declared amount if lost.
Usage
Cash may legally be mailed only this way in Japan; putting bills in an ordinary letter is not allowed. The envelope has room for a short note, and it is common for family gifts such as お年玉 or wedding money sent from far away.