AMEX is offering $10 off on Amazon if you change your payment card to an AMEX.
One time use per account.
I originally found the link haphazardly browing through on incognito mode. URL worked on my account once I had logged in. My default card was Discover due to the 5% cash back through Dec 31.
Purchases must be sold from Amazon directly. I tested it out myself. The offer appears as "AMEX PROMO" in cart. You must use an AMEX card to check out, as well.
Terms of the offer:
This promotional offer is available only to customers who were served an advertisement and to purchases made with an eligible American Express® Card. Complete the following actions to claim the offer:
Click on the ad promoting this offer.
On the 1-Click landing page, sign in to your Amazon.com account.
Change your 1-Click default payment method to an American Express Card by the earlier of 11:59 p.m. PT on November 25, 2020 or while supplies last.
When prompted, ensure your American Express Card’s billing address is accurate by either confirming your billing address or adding a new billing address.
After steps 1-4 are completed, the $10 credit will be applied to your Amazon.com account.
To redeem the credit, make subsequent eligible purchase(s) with your eligible American Express Card by 11:59 p.m. on December 25, 2020.