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registered mail; registered post; registration (of mail)
(putting on) record; writing down; noting down; registry
大切な書類は書留で送ることにした。
I decided to send the important documents by registered mail.
書留なら届いたか確認ができます。
With registered mail, you can confirm delivery.
書留で送ると少し料金が高くなる。
Sending by registered mail costs a bit more.