Midknyght's Comments

I found the 'Discarded Letter' on my 2nd run- sitting on top of a crate before I fought Wandering Phantasm. I clicked on it and it disappeared... didn't put anything in my bags.  

After I fought Nefarious Terry, instead of going into the final area... I took a right into that little nook with the mailbox at the end.  I clicked on mailbox and nothing seemed to happen.  When I turned around to leave, Postmaster Malown was standing right there in front of me.

It just dropped for me in 10-man Normal, so I can vouch that you don't need to be in Heroic mode.