SJP ON THE SELLING FLOOR: Sarah Jessica Parker was working the sales floor of her pop-up shop on West 52nd Street on Monday. With customers milling around the store looking at footwear and handbags, Parker was asking customers, “How can I be helpful?” and “Who haven’t I spoken to yet?” She even got down on her hands and knees to help customers try on shoes. The pop-up, which opened for the holiday season, returned to its location on Monday at 6 West 52nd Street. Parker has alluded to the store opening in an Instagram post on Monday, inviting one and all to drop in and see the SJP Collection. “The store’s open until further notice!” she posted on Instagram.