NEW YORK (AP) — Thrive Market, a four-year-old online membership-based natural and organic food retailer, has successfully filled a niche modeled after Costco but just for healthy items.

CEO Nick Green, along with his two co-founders Gunnar Lovelace and Sasha Siddartha, saw a big opportunity to make healthy food more accessible for middle-class American shoppers who found Whole Foods too expensive and were overwhelmed by the increasing choices at discounters.

Members pay an annual membership fee of $59.95 to enjoy a 25 to 50 percent discount off traditional retail prices. For every membership...