@Golliathe people paying a monthly sub for their mule accounts is common.
Re the Mail system and mules I've never seen this discussed by the devs.
I don't expect a mail system anytime soon, but why would a player go to a "mailbox" and sort through various items to send to various mules. Then the mules have to go to the mailbox and pick them up? Complete waste of time, imho, when it's simply easier to log the mule on and drop those items onto the mule.