As stated before, the game is free. So paying for "the game" doesn't "include a month sub", a month sub is all you pay for. If any store asks more, they're ripping you off (I have seen some here do that).
And yes, being a sub is the only way to send mails with COD. Non-subs can receive them though.
COD is adding an item and then the user can only accept it upon paying a fee (then given to the mailer). It's a way to make sure you're not ripped off, and the other person really has to pay you to get his item(s)...