No trick, it should just be a case of buy the item from the optional items page and they'll send you a mail in game with the item.

However, there is a limit on the number of Square Enix mails you can receive (20 I believe, different to the limit of number of mails you can receive from other people) so if you still have lots of their previous mails (the ones giving you veteran rewards for example) new ones can't come in until you delete some of the old ones).

So when you check your mail, I think somewhere down bottom right it says something like ../20 which is relating to the Square Enix emails. If you're at the cap, delete some old the one ones and hit the button in the top right of the screen for force checking new mail.

Hope that helps!