Hi there. Exotic Ciphers do not go to the Postmaster. If your inventory was full or you already had a Cipher in your inventory, that one is lost and cannot be returned. To get another, you will have to complete the quest again next week when Xur returns. When turning in the quest to Xur, always make sure you do not already have an Exotic Cipher in your inventory and that you have space to receive one or it will be lost.