What is the last sentence Ennis utters in the film while looking at the shirts.
It sounds like four words or five..the last two being "I swear."
Perhaps it's in the short story.

I have not seen the movie, but here is the ending of the story by Annie Proulx:

====================================================================
  "Jack, I swear -- " he said, though Jack had never asked him to 
   swear anything and was himself not the swearing kind.

Brokeback Mountain
http://www.wesjones.com/brokeback.htm
====================================================================

Here you'll find an interesting thread discussing the line's meaning:

Brokeback Mountain - What does Ennis swear?
http://www.annieproulx.com/forum/phpbb2/viewtopic.php?t=57&

If this would be fully satisfactory as an answer, please let me know.

thanks.  That's a good start.  Will keep trying to get the movie line as well.

OK. I hope someone can find more info for you.

If you're a member of the Internet Movie Database, these threads may
be of interest to you:

http://imdb.com/title/tt0388795/board/flat/32227838

http://imdb.com/title/tt0388795/board/flat/32224910

I just got back from watching that movie and he says "Jack, I swear."