The Amex Platinum $200 airline fee credit, the Amex Premier Rewards Gold's ("PRG") $100 airline fee credit, and the Amex Hilton Honors Aspire's $250 airline fee credit are supposed to apply only to incidental charges like checked bags and in-flight amenities, not airline tickets. However, buying a gift card on AA, Delta, or SW up to certain denominations is known to trigger the airline credit reimbursement. For Delta, you can buy up to a $50 gift card in one transaction and do it in however many separate transactions as you need to max out the credit; for AA, the maximum gift card value per transaction is $100; and for SW, buying a $200 gift card works on the Platinum's $200 travel credit, and I believe you can even do a single $250 gift card on the Hilton Aspire.
Two caveats: (1) the airline fee credit only triggers when you are purchasing a gift card for the airline you selected in your member benefits, so you need to make that selection when you first get your card, in January, or leave it from year-to-year. This is a calendar year benefit that resets every January 1. You may be able to chat/contact Amex to request to change your airline selection if you "forgot" to change it in January and you haven't yet used any of the credit for the year. (2) Of the airlines selections on Amex, this only works on AA, Delta, and SW. I recall United used to have a TravelBank gift card registry that worked, but it went down in late 2017. Hawaiian Airlines gift certificates used to work, but that also changed sometime in late 2017. Because things can change, it's always recommended to check the latest data points on places like FlyerTalk before buying a gift card if you expect to see a reimbursement.
One last thing, there are scattered DPs that cheap flights, like under $100, may get reimbursed under the Amex airline fee credit, but they can be hyperspecific blips to the more general pattern above.
