Just before that passage Sammy "saunter[s] into the electric eye in [his] white shirt that [his] mother ironed the night before, and the door heaves itself open, and outside the sunshine is skating around on the asphalt." A few things lead up to the "hard world" he finds past the electric eye and in the "sunshine skating." His mother had before cared for him, and that will no longer be. The electric eye symbolizes the mechanistic world into which he now walks and the childlike world he leaves behind. In the mechanistic world of adults, he has less effect on the outcome of actions: he does not push the door open but something else causes it to open, and the "eye" connotes scrutiny upon him by someone with sight more powerful than his own. And it's hot out there on the asphalt while cool in the store--the asphalt itself suggesting something barren and bleak. No wonder he concludes, looking upon this, how hard the world will be on him hear after. He walks across a threshold, literally and figurative, from one life into a another.
I would also add that the magnitude of his decision finally hits Sammy when he realizes that the girls are already gone. In that final moment he realizes what he just threw away. It is not like he has any job opportunities knocking down his door. Remember, Lengel gave Sammy that job as a favor to his family. And since part of what Updike is trying to show us is the disparity between the social classes with Queenie and her friends and the store workers, part of Sammy's final realization is that he learned a hard lesson for the working class. Also note that while Sammy is quick to criticize everyone from his co-worker, Stokesie, to the female customers, at least they all have roles and are serving purposes. Sammy, though, is really serving no purpose other than criticizing others and being unhappy with his station in life. Does he look back differently at his job, Stokesie, Lengel, and those customers when he is now out of work and realizes "how hard the world was going to be for me hereafter"?
In deciding to quit his job over the girls, Sammy has abadoned the innocence of his youth. He has seen the prejudice of his boss, and seen that his own decision has been based on a bad premise (that he would impress the girls, who didn't pay attention). The world is going to be hard because he has come of age, and must deal with adult issues, adult feeling, and adult consequences.