This reputation item became available during The Sunken Curse curse event from July 11 - 25, 2018 and is not expected to return. The letter can only be purchased with 30 doubloons earned during the event. Using it adds +1 rep to the player's Merchant Alliance reputation. The vendor is the Bilge Rats representative Duke who can be found at the Tavern.
Once word of your deeds gets around, it'll raise your standing with the Merchant Alliance.