Air:Baggage fees
Fees/ taxes when booking a mileage ticket
Pet travel fees
Seat upgrade fees
Airline
e-GCs: best $100 increments or smaller
Delta-$50 increments.
Don't buy on mobile site! Southwest
Alaska
American Airlines No longer works as of Feb 2019
Other:MPX (mileagePlus X app )United gift registry:
Make united your airline of choice for your platinum.
Create a united gift registry. Create a goal for the registry.
Add funds to the registry in increments of $100 or less using your platinum card. Get reimbursed by AMEX.
DDMS
https://www.dansdeals.com/credit-cards/used-2019-amex-airline-fee-credit-will-use/Discussion about how to use more than 3 Delta GC on 1 ticket:
https://www.flyertalk.com/forum/delta-air-lines-skymiles/1888784-possible-use-more-than-3-gift-cards-one-booking-2.html