Surface mail is transported via truck, rail, and ship, rather than by plane, as in this FedEx Ground truck.
Surface mail, also known as sea mail, is mail that is transported by land and sea (along the surface of the earth), rather than by air, as in airmail. Surface mail is significantly cheaper and slower than airmail, and thus is preferred for large or heavy, non-urgent items – it is primarily used for packages, not letters.
The term "surface mail" arose as a retronym (retrospective term), following the development of airmail – a term was needed to describe traditional mail, for which purpose "surface mail" was coined. A more recent example of the same process is the term snail mail (to refer to physical mail, be it transported by surface or air), following the development of e-mail.