Complex sentence: The retail organizations that distributed the stamps (primarily supermarkets, gasoline filling stations, and stores) bought the stamps from S&H and gave them as bonuses to shoppers based on the dollar amount of a purchase.
Split into simpler sentences: The retail organizations that distributed the stamps were primarily supermarkets, gasoline filling stations, and stores. They bought the stamps from S&H and gave them as bonuses to shoppers based on the dollar amount of a purchase.