express mail; special delivery mail
Refers to mail sent by express or special delivery service, faster than ordinary mail.
この書類は速達郵便で送ってください。
Please send this document by express mail.
速達郵便 is the full, formal term for express mail, and 速達便 is its everyday abbreviation.
express mail service; PD PR
Compound of 速達 (express delivery) and 郵便 (mail, postal service).