What is the climax of "The Possibility of Evil" and why?

The denouement of "The Possibility of Evil" is what we would call a climactic ending. The ending is climactic because it is unmistakably life-changing in nature. Miss Strangeworth's habits are no longer a secret; someone is privy to her actions. For the woman who has never contemplated the "possibility of evil" in her life, the revelation that her position has been reversed is terrifying. The hunter has become the hunted. The ending is climactic because it is stark, shocking, and ironic: Miss Strangeworth's life is forever changed and impending events are beyond her ability to control.
We are told that Miss Strangeworth sent out three letters during her last outing. The envelopes were pink (to Don Crane), green (to Mrs. Harper), and blue (to Mrs. Foster). The Harris boy retrieved what appeared to be a pink envelope and presumably delivered it to Don Crane. However, the envelope (and letter) that returns to Miss Strangeworth is green in color.
We know that the green envelope was sent to Mrs. Harper. Here's the quote that describes what Miss Strangeworth wrote to Mrs. Harper:

She selected a green sheet this time and wrote quickly: Have you found out yet what they were all laughing about after you left the bridge club on Thursday? Or is the wife really the last one to know?

The phrase "the wife is usually the last to know" refers to how a woman often discovers her husband's affair after everyone else. The quote above is intriguing because of how it characterizes Miss Strangeworth as an omniscient purveyor of justice in her community. Yet, the climactic ending turns this characterization on its head. Instead of being the powerful, righteous matriarch presiding over a wayward community, Miss Strangeworth becomes a terrified old lady at the end of the story.
Someone else knows about her secret life. The reader is left to contemplate the possible identities of the one who has discovered Miss Strangeworth's secret. Is it Don Crane, since the Harris boy delivered his letter (and very likely revealed that Miss Strangeworth sent it)? Or is it Mrs. Harper, the one Miss Strangeworth believes is the last to know about her husband's affair? After all, the letter that was returned to Miss Strangeworth was green. Or, even more intriguing, can it be that more than one person knows about Miss Strangeworth's secret?
Whatever the answer is, the ending is a climactic one. Miss Strangeworth's life is forever changed and not in a very pleasant way.

In literature, a climax is a moment in which the tension in a story reaches its peak. It is also helpful to think of the climax as a turning point in a story, a moment which leads directly to the resolution.
In "The Possibility of Evil," the climax occurs when Miss Strangeworth goes to the mail slot at the post office and accidentally drops one of her poison pen letters. Instead of posting this letter, it is picked up by the Harris boy who sees that it is addressed to Don Crane. Thinking it might be important, the Harris boy decides to deliver it to Don Crane himself.
This moment is climactic because the audience realizes that once Don Crane receives the letter, Miss Strangeworth's secret will become public knowledge. Everybody will know (and have undeniable proof) that she is the author of the poison pen letters. What really adds to the tension in this scene is that Miss Strangeworth has no idea that her letter has been intercepted by the Harris boy. As a result, she is powerless to stop this chain of events.