Yes, you can mail characters on your account mail, and a bonus is that when you send mail to characters on your account it is instantly delivered. All you have to do is go to a mailbox (which are found near inns, banks, auction houses) right click it, click the send mail tab, type in the name of the character you are sending mail to, drag over whatever you are sending, and click send. If you are sending money you have to type in something in the subject line too. But that's it, it's super easy.