Wow, interesting thread.
Great question, Patriot, but I'm going to have to agree with No Reins; I have a strong impression for some reason that Jack has been dead for some time--months perhaps--before the postcard is returned and the robotic recital by Lureen.
At the very least, enough time has elapsed for a funeral to be arranged, cremation to occur, burial in Texas, message to Wyoming, ashes sent to Wyoming.
This is just impression/opinion, but it seems to me that mail to a deceased person--which would include important financial documents, bills, etc--does not get marked "DECEASED" :'( and simply returned immediately upon death. If a family is there (as is the case with Jack) then the mail will continue to be delivered so that all the loose ends may be tied up. Then, after a period, the mail is stamped and returned.
Also, I can't imagine, Ennis being Ennis, that he would send a postcard months in advance. I get the impression that Jack usually initiated contact, but in this instance Ennis realized he'd better write something since he hadn't heard from Jack and he was concerned that maybe Jack was "still sore from that (psychological) punch" he'd gotten in the parking lot.
And I agree with others that it WAS Randall that got Jack killed, but not intentionally. If anyone premeditated Jack's death, I'd say it was some of Lureen's relatives. If you see the movie only and don't read the story (in which LD had died), it certainly could have been LD Newhouse. Otherwise, it was just some guys on a spontaneous hate rampage.