Kafei's Letter

From Zelda Wiki.org

In Majora's Mask, Kafei's Letter is given to Link from Anju at the Stock Pot Inn in East Clock Town. Link must deposit it in any Postbox to be delivered to Kafei, who will then run outside to grab the letter when it is deposited into the mailbox at the Laundry Pool, allowing Link to go inside the back room of the Curiosity Shop. It is one of the items used in the long trading/action sequence between these lovers that is done to receive the Couple's Mask.