The Clue of the Left-Handed Envelope (Third-Grade Detectives)

WHO SENT AMBER LEE AN ANONYMOUS LETTER?
Everyone loves the new teacher, Mr. Merlin. Mr. Merlin used to be a spy, and he knows all about secret codes and the strange and gross ways the police solve mysteries.
Now there’s a mystery in their very own class — someone sent Amber Lee Johnson an anonymous letter. Noelle and Todd want to solve the mystery before anyone else. But first they have to figure out Mr. Merlin’s clues. How can a bunch of envelopes tell them who sent the letter?