Why does Miss Strangeworth mail her letters at the post office after it has closed?

Miss Strangeworth mails her letters after the post office has closed to avoid being seen by anyone in her small town. She wants to keep her identity hidden while sending her malicious letters, which she believes serve a moral purpose by exposing the perceived flaws and wrongdoings of her neighbors. This secrecy reflects her desire to maintain her reputation as a respectable citizen while engaging in her covert campaign of judgment.