"Costco's credit card policy is all about saving money. It negotiates a partnership agreement with a single payment network in exchange for much lower processing fees. As part of its current deal with Visa, it pays transaction fees of only 0.4%. By saving on fees, Costco is able to keep prices low."
https://www.fool.com/the-ascent/credit-cards/articles/why-do...
They actually secure a very nice low rate via their credit card policy