Clue by Clue: The Letter Looter Solution
Lisa wanted to send proof of the affair to the man’s wife. However, he didn’t know her address. She looked the address up online. She accidentally found another Dr. Mary Williams in the same town. She sent the letter to this Mary Williams, not the man’s wife.
The man killed Lisa to stop her from telling his wife about the affair. Then he waited for her letter about the affair to come home. He got the mail before his wife could see the letter. After five days, he realized something was wrong. He figured out Lisa had probably sent the letter to the wrong person, found the other Dr. Mary Williams, and stole the letter.