Sign into your American Express account and you may find this offer under the “My Offers” tab (that’s all I had to do).
You can also tweet #AmexAmazon to register. You need to have synced your American Express card and twitter account.
Be sure to register right away, before the offer disappears. If you don’t have an immediate $75 Amazon purchase, but want to lock in the $25 savings, just buy a $75 Amazon gift card. Then you can split the purchases of into amounts below $75, or make the purchase(s) at any time in the future.
This is another example of American Express’ new partnership with Amazon.com that can even get you free Amazon Prime.
(HT: Frugal Travel Lawyer)