How to Use Amex Airline Credits to Buy United Tickets
Update: This may not be working, if you choose to try, my recommendation would be to call first to confirm that applying the gift value to the travel bank is still an option.
The Amex Airline credit reimbursements are pretty restrictive when it comes to the types of charges that qualify. Gift cards are a redemption favorite because you can use them to pay for airfare and they don't expire. United flyers used to be able to use the Gift Registry option to add funds to their travel banks and trigger the credit but that has since closed. Good news, there's still hope for you.
I noticed while doing research on United's site for my InFlight WiFi Guide that United sells WiFi subscriptions which should trigger the Amex credit. (This Flyertalk thread reports a few affirmative DP's.) I also noticed that if you receive a WiFi subscription as a gift, the recipient can choose to redeem it for a subscription or apply the value to their travel bank for future use.
United's site states “The gift giver may have sent you a gift activation email or printed a gift receipt for you — you'll find your code there. After you enter your code, you can choose to either activate your subscription or deposit the value into your United® TravelBank to apply toward future travel or other subscriptions.”
United Monthly subscriptions are available to purchase for $49 and $69.
The option to add the value of the WiFi subscription to your travel bank is effectively the same as purchasing a gift card (aside from the inability to regift the value). Funds in your Travel bank can be used to purchase tickets which is a nice workaround to the airfare restriction.