A friend of mine used to own an Esso franchise garage near Box Hill on the A4.
He reckoned to sell petrol at cost and made all his money on chocolates, newspapers, cigarettes, batteries, sundries. Shop turnover was twice the fuel turnover.
He put the nearby two bigger, competing fuel-company-owned garages out of business with his lost leader, fuel.
This was ages ago and the goods are different these days, but the principle still holds.