post office worker; postal employee
Refers to a person who works at a post office, typically in a clerical or counter role.
The post office clerk kindly explained the procedure to me.
郵便屋 refers specifically to the person who delivers mail, while 郵便局員 is any employee working inside a post office, not necessarily a delivery person.
Compound of 郵便 (postal service), 局 (office), and 員 (member/staff).