I think it would be nice to have an added function to mail with attachments. A COD (Cash on Delivery... well, Cr or CE on delivery)
You create a mail that has an attachment and put the fee in the COD location. When the recipient of the mail opens it, they can read the body of the mail but they cannot retrieve the attached item(s) without paying the COD costs by hitting an "accept COD" button. If they decline the COD, the item is delivered back to the sender. If they accept, the payment is delivered back to the sender.
Functionality for both Crowns and Crystal Energy would be optimal.
I think it would clean up interactions like seeing that recipe a guildie wanted or completing transactions from the Bazaar forum without having to meet up in game.
While I suport the idea in theory, I would much rather them have to enter the crown/energy cost manualy. Many folks simply hit accept and deleat on mail, I can definatly see a spree of scaming with people CODing scrap metal for 20k crowns to have people accept by rote if this hit live as you describe. Thus as a saftey mesure the accept buton will be un available untill the reciepiant has entered the prescribed amount of cr/ce.
Adding a rebound setting would also be desired. By which i mean if someone neither accepts or declines and instead lets it sit in there mailbox, it will automaticly return to sender after a time period (default of a week, but setable by sender).