What does the last sentence of "A & P" mean?
Your question points towards the importance of paying particular attention as to how authors choose to end their works and how the ending in particular relates to the overall theme or message of the story as a whole. Let us analyse therefore the last sentence of this great short story:
His face was dark grey and his back stiff, as if he's just had an injection of iron, and my stomach kind of fell as I felt how hard the world was going to be to me hereafter.
Sammy has just made a stand for his own personal beliefs and values by walking out of his job because of the way his boss, Lengel, treated the girls who came into the store in their bikinis. As he walks away, Sammy looks back and describes Lengel, emphasising his stiffness, as if he "just had an injection of iron." This triggers off an epiphany for Sammy as he recognises how hard life is going to be for him if he carries on choosing to follow his own values and principles, which will put him in conflict with the world's values and principles, perhaps captured in the unyielding description of Lengel's back. Thus the ending is important because it captures the coming-of-age nature of this story and the age-old dilemma of being true to ourselves or following the ways of the world and the consequences of such a decision.