surface mail; land mail
Refers to postal mail sent by land or sea, as opposed to airmail (航空便). Commonly used in postal contexts.
この荷物は陸便で送ります。
I'll send this package by surface mail.
航空便より陸便のほうが安いです。
Surface mail is cheaper than airmail.
Compound of 陸 (land) and 便 (mail, service). The term likely arose as a counterpart to 航空便 (airmail) to distinguish surface transport.