The top five grocers are investing in buy online/pick up in stores, along with home delivery of online orders. Inc.’s acquisition of Whole Foods Market gives the e-commerce giant its best chance yet to devour the food business.  For its part, Amazon is wasting no time integrating Whole Foods with its popular Amazon Prime program. Online food sales are still a small part of the $700 billion-plus U.S. grocery market. But, even before Amazon began melding Whole Foods’ network of 465 stores in with its own business, grocers were waking up to the need to leverage store locations as a competitive advantage with online shoppers.