The P.O., or post office, where the character Sister works is symbolic. She works in a tiny post office in China Grove, in which her family is the main family. After deciding to leave home, Sister has redesigned the post office in the way she likes it, and she has placed the possessions she has taken from home, including her radio, ironing board, and sewing machine, into the post office.
Sister is a character who has disliked the intrusion into her life of elements from wider world, including her sister, Stella-Rondo, and the child who Stella-Rondo claims is adopted. Stella-Rondo left the town and has returned, much to Sister's dismay. It's ironic that Sister has taken up residence in the P.O., where she can pretend her family has vanished from the face of the earth. There, she can almost pretend that there is no outside world, even though the post office is the place to which missives and messages from outside the town arrive. The idea of isolating oneself at the post office is ironic, as the post office is the conduit to the outside world, but the post office is also where Sister barricades herself away from the rest of the world. Her position in the P.O. represents her conflicted existence in which she wants to block out intrusions from the outside world but is still affected by the wider world.