Something just occurred to me watching the opening credits tonight (repeat of when Carlos comes home early and Gabby nearly gets caught with John).
When the girls find the blackmail note they give it to Paul, who later finds the same stationery in Edie's organizer. He learns that the stationery came from Mrs. Huber's house and he assume's the note was written by Mrs. Huber.
All this time we've been trying to figure out why Felicia has such a connection to Zach and why she is so concerned with his welfare......... PAUL KILLED THE WRONG SISTER!!!! I'm guessing that Felicia has the same stationery and gave some to her sister...or on a previous visit to her dear sister Felicia met Paul and Mary Alice and recognized them.
FELICIA wrote the note and was blackmailing Mary Alice.