a million dollar 16 pump gasoline station is a lot of scratch for "customer loyalty".
Other gas stations don't have costco's buying/negotiating power, costco does nothing without it being profitable.
COSTCO could not get the deal they wanted with American Express, so amex is being ejected from COSTCO next April. AMEX admited that COSTCO account for %16 of their total business. That's purchasing power.
guaranteed COSTCO is squeezing every penny out of their gasoline supplier, it's like "hey, play ball with us or your gone". Sure, the gas station brings people to an already crowded COSTCO, but not to the extent you surmise.