Retail gasoline is a natural extension of the Wal-Mart business model. Sourcing the product through a distributed supply chain with technology-enhanced logistics gives an advantage to a company that can use technology and distributed information systems to lower its costs. Retail margins on gasoline sales are only around 2 percent anyway, which is why the traditional gasoline retailers (including vertically integrated and franchised and independent) have diversified so heavily into convenience items to up their margins on goods that are complements in consumption with gasoline. I can also see how signing a contract with Wal-Mart would be very attractive to a refiner, even if Wal-Mart follows its famous hard-nosed negotiation strategy.
There is another large retailer called Meijer in the Midwest that has been following this strategy for gasoline for almost a decade, and this getting economies of scope over small margin items and selling lots of them seems to be working for them too.
In fact, there’s been some criticism of Meijer and some accusations that they use gasoline as a loss leader to attract business. I do not believe anything has ever come of that accusation, and that it’s probably as unfounded as many accusations are of collusion and price gouging in the gasoline industry.