if they gave the 3rd retainer an ability that was well deserving of the monthly fee people would pay for it and square could still allow players to mail their alt's. Say with the 3rd retainer it could learn a crafting job, or it would have access to other allowances like an 24 hour 4 token spirit binding venture where it would spirit bind gear for you. there are innovative ways to get players to spend extra money while pleasing the mass and allowing them the simple ability of sending mail from one character to another. Most people would probably keep the 3rd retainer anyways just so they wouldn't have to log out log in find item run to mail log out log in go to mail.