n. — Administration, informal, diminutive
a letter carrier or, occasionally, another type of postal worker.
Type: 2. Preservation — Postie has been preserved from British English and it appears to be of Scottish origin (see OED-3, s.v. "postie", and W-3, s.v. "postie"). It is currently used in many former British colonies, although it appears to have little currency in the US. Postie is a term of endearment, a diminutive that is almost always used in a positive way.
See also COD-2, s.v. "postie", which is marked "Brit. & Cdn informal".