As American Express and Citigroup clashed over which issuer would get the coveted Costco Warehouse credit card business, one area remained undisputed and out of their hands: Payment processing for Costco's small business clients.

That role is held firmly by Elavon, a unit of U.S. Bank, which will mark its 20th year in April as the provider for small businesses that seek payment processing as part of their Costco memberships. First Data has a similar role with Sam's Club, providing the payment processing for more than a decade for members of the Walmart-owned warehouse retailer.

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