Costco makes ~75% of their profit from memberships, gas is like their iconic $5 rotisserie chicken - sell it at cost or a small loss as a incentive to buy and

Costco makes ~75% of their profit from memberships, gas is like their iconic $5 rotisserie chicken - sell it at cost or a small loss as a incentive to buy and maintain membership. Gas prices are posted outside so it's a particularity public ad for costco membership and as such they have a strong incentive to push the price as low as feasible.