You can turn back in unwanted Armor Items for 5 Society Trade Tokens.
Lore & Dialog
Society Breastplate Armorsmith tells you, "You must be at least a Lord within the Society to trade me an Armor Writ for my key to this chest, sorry."
Society Breastplate Armorsmith tells you, "If you bring me an Armor Writ from the Quartermaster, I can give you a key for my chest."
Society Breastplate Armorsmith tells you, "You can only receive one key in exchange for an Armor Writ every day, so choose wisely."