Originally Posted by
xliioper
There's no requirement you need to actually use your DL Amex card to purchase the awards. The DL Amex first bag free checked luggage benefit also does not require you purchase ticket with your DL Amex card (it will be applied to any DL operated and marketed flight with your name and Skymiles number on it).
Unlike (I think) with that benefit, though, the terms do specifically say you must pay with the card:
Discount only applies to the mileage portion of the Award Ticket fare and does not apply to applicable taxes and fees which must be paid using the Eligible Card.
(BTW, I love the Delta baggage fee waiver vs. the United one for that very reason. Of course I'm sure it is, in part, because Amex knows much of the value of the platinum comes from the 5x on flights.)