regular mail; surface mail; standard delivery
Refers to ordinary postal service, as opposed to express, airmail, or registered mail. Often contrasted with 速達 (express) or 書留 (registered).
I'll send these documents by regular mail.
Please send it by regular mail, not express.
書留 is registered mail, providing proof of sending and delivery, unlike 普通便.
航空便 is airmail, typically used for international mail, while 普通便 can be surface mail.
Compound of 普通 (regular, ordinary) and 便 (mail, service).