Update: This popular deal is still available for select accounts.
Amazon offers Select American Express Membership Rewards Cardholders: 20% Off Your Purchase when you use at least 1 Membership Rewards Point and follow the steps below. Thanks krishnakotapati
Note, offer eligibility will vary by cardholder. The maximum discount is $50. There is another ongoing Amex Promotion for $30 Off $60, check out this Frontpage deal for the details (the promotions do not stack).
Click here to link your American Express Membership Rewards account to your Amazon account
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Editor's Notes & Price Research
Please see offer page for full terms and conditions. This is a limited time offer. The promotion will end on the earlier of (a) 11:59 PT on 07/16/2019, or (b) the time at which offers worth $5,000,000 have been redeemed.
If you are linking your Amex Rewards account to Amazon for the first time then it can take a few days before you may become "eligible" for the promotion. You can also try removing and re-adding the card as a payment option to see if this will prompt "eligibility", try the link again in a few days to see if this works. These offers can open up over time so you can always check later if it does not work out this time around.
This offer will work on most 3rd party gift cards (both physical and digital), but not Amazon gift cards. -qwikwit