The in-game mail (note: ' is default English key-binding for mail) has an attachment, which you have to Take the attachment which places a the item in your inventory (miscellaneous type).

Note the in-game mail is sent to your first character. Do not delete the mail message or the character before retrieving the items.

Edited: Deleted the incorrect reference to containers (i.e. urns), the order bonuses are "direct" and are of type miscellaneous.