Stoicism is showing and indifference to either pleasure or pain -- i.e., taking life as it comes. Ennis tries to show this many times, but we do know that it is all a facade, by the way he breaks down in the alley, or the way he breaks down in the final confrontation.
Relating to the scene in the closet with the shirts, we see the true Ennis, engulfed in a quiet sea of sadness. That's what he truly was inside, and not the stoic he wanted people to see.