Answer: By charge card. An American Express card to be exact. Delta’s charge card has a credit limit of $612 million, and just like you and me, it’s paid monthly. Though to be fair, it probably does not look like the card pictured or earn SkyMiles with every purchase.
From Delta’s 10-K Annual Report: “Fuel Card Obligation. In December 2011, we also obtained a purchasing card with American Express for the purpose of buying jet fuel. The card currently carries a maximum credit limit of $612 million and must be paid monthly. As of December 31, 2011, we had $318 million outstanding on this purchasing card, which was classified as other accrued liabilities.”