I have DL as my selected airline for my Amex Business Plat and Hilton Aspire cards, $200 and $250 (resp.) are reimbursed annually from each--normally airfare purchases don't qualify. In the FT Amex forum there are work-a-rounds using DL gift cards; pre-2019 DL gift card purchases counted, now it is a little "trickier"
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PS: In my OP, I deselected my AMEX and chose a VISA debit card to purchase a DL gc, I then restored my Amex as the primary.
RE: the AMEX Plat/Aspire airline fee credit, you don't need to start with a gift card to use the workaround. An eCredit will work just as well.