Good point Mandy.....
I have to admit, that I was one of those first time viewers of the movie that did not see "DECEASED" stamped on the postcard.
It wasn't until he was making the phone call that it hit me what had happened.
Was it a case of Ennis justifying what he had always thought, that they could not live together safely, we see that when his own thoughts of what happened are contradicting what Loreen is telling him. Was he half expecting something like this to happen?
I think the reason it would be so much of a wallop to the story line and us viewers, was the finality of what we were all hoping might happen, the "they lived happily ever after" conclusion was not going to happen. But as far as Ennis was concerned, they were never going to live happily ever after, so when Jack died, of course heartbreaking, it was also the end of the SECRET, the double life Ennis had been living. Look how easily, he could phone Laureen, even talk to her about Brokeback Mountain, for the first time admit he was a friend of Jacks. And then to go to Jack's home, something he would never have done while Jack was alive.
Finding the shirts, was Ennis's wallop....here it was, the two of them together as " lovers", in Jack's family home. It was then, that Ennis mourned the loss of the love of his life, and taking the shirts, and that nod to Jacks mother was the first time he acknowledged to anyone the feelings he had for Jack.