How does Miss Adela Strangeworth betray herself in "The Possibility of Evil"?

Miss Adela Strangeworth writes her anonymous letters on various-colored papers and puts them in matching-colored envelopes. In the scene in the story in which she is writing three new letters, 

...she wrote on a pink sheet: Didn’t you every see an idiot child before? Some people just shouldn’t have children, should they?

Then on a green sheet she writes a note intended for Mrs. Harper.

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?

It is important for the reader to remember the colors of these letters in order to understand what happens at the post office.

She addressed an envelope to Don Crane after a moment’s thought,… using a pink envelope to match the pink paper. Then she addressed a second envelope, green, to Mrs. Harper. Then an idea came to her and she selected a blue sheet and wrote: You never know about doctors. Remember they’re only human and need money like the rest of us. Suppose the knife slipped accidentally. Would Doctor Burns get his fee and a little extra from that nephew of yours? She addressed the blue envelope to old Mrs. Foster, who was having an operation next month.

After finishing her three letters she eats lunch and then takes a nap. When she wakes up she sets off on her regular evening walk with the letters in her purse. She never mails her poison-pen letters at any place but the post office. The outside of this building happens to be a hangout for children and teenagers. Miss Strangeworth overhears two of her victims talking and stops to eavesdrop on the conversation. She has made both of them unhappy by writing to Linda Stewart's parents suggesting that Dave Harris is carrying the romance with their daughter beyond the usual teenage hugging and kissing. 

“I can’t tell you, Dave,” Linda was saying – so she was talking to the Harris boy, as Miss Strangeworth had supposed – “I just can’t. It’s just nasty.”

The mail slot for depositing letters after hours is in the front door of the post office. Miss Strangeworth opens her purse and slips two of her letters inside. But she doesn't notice that the third letter doesn't go all the way through the slot. It falls to the ground. Ironically, Dave Harris whom she has caused so much unhappiness, picks it up and decides to hand-carry it to the addressee, Don Crane. The reader may not realize immediately that Don Crane is the husband of Helen Crane, the young mother who told Miss Strangeworth earlier that she and her husband were worried about their six-month-old daughter's apparent slow development. But the color of the envelope will reveal everything.

Sadly, holding hands, they wandered off down the dark street, the Harris boy carrying Miss Strangeworth’s pink envelope in his hand.

The reader knows that Dave will tell Don Crane that Miss Strangeworth accidentally dropped the letter at the post office. And Dave's girlfriend will be with him to confirm it. Miss Strangeworth will never know who it was that chopped all her precious rose bushes to pieces and sent her the anonymous letter reading:

Look out at what used to be your roses.

How does Miss Strangeworth get caught in "The Possibility of Evil"?

Miss Strangeworth gets caught when she accidentally drops one of her colorful but nasty notes outside the post office. She goes to put her letters into the slot to be mailed the next day, and two of them fall inside as they are supposed to, but one letter—the one addressed to Don Crane—falls to the ground outside. A teenage boy, Dave Harris, sees it fall, picks it up, and calls after Miss Strangeworth, but she does not hear him, because she is preoccupied with considering whether or not she should write a letter to Dave's father, alerting him to the boy's "potential badness."

Ironically, though Miss Strangeworth has already maligned Dave Harris to the parents of his girlfriend, Linda Stewart, and though she is considering sending a similar letter to Dave's father, it is actually Dave's act of kindness that results in the revelation of the nasty letter writer's identity. When Dave sees that the letter is addressed to Don Crane, he says,

Maybe it's got a check or something in it and he'd be just as glad to get it tonight instead of tomorrow.

Though Linda thinks it is pointless to "do anyone a favor," because no one seems to care about them, Dave decides to hand deliver the letter anyway because, he thinks, it might contain some good news the Cranes would like to have that evening.

Readers can assume, then, that Dave Harris tells the Cranes who dropped the letter at the post office—Miss Strangeworth—and this is how she gets caught.