Update 2: Direct registration is now closed. Registration for this promo through twitter is still open but you may want to hurry before that gets closed too.

Update: You can register your American Express credit cards via this link. Additionally, you can also register all of your “Additional User” cards for this promo! Lock in the savings now by buying Amazon gift cards.  
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American Express has a new promo. Get a $25 Statement Credit with any $75+ Purchase at Amazon! Officially this offer is valid through December 31, 2013. Though you should probably add this offer to your card ASAP, as this offer will probably fill up before then.

To register for this promo;

1. Sync your Amex card to your Twitter account HERE
2. From your Twitter account, tweet #AmexAmazon to add the offer to your synced Amex Card.
3. Make a single purchase of $75 or more with your synced card at Amazon by December 31, 2013.
4. Receive the $25 within up-to 90 days after your purchase – (though usually a lot sooner)

See T&C’s here

Note: Make sure that you’ve received a “@ reply” message from @AmexSync stating that this offer has been added to your synced Card

No Twitter? You may be able to register by logging in into your americanexpress.com online profile and checking under “offers for you”, or on Facebook or on Foursquare,

Not sure what to buy? Just buy an Amazon gift card! They never expire and you can lock in the savings now.