Saw this offer popped up when I tried to check out:
Get $15 Off When You Redeem Citi ThankYou Points.
- Click on the button to activate this promotion credit and apply points to your next order. Note that you will NOT receive the credit unless you click on this button.
- ADD $50 OR MORE of eligible products sold and shipped by Amazon.com to your shopping cart.
- Your Citi ThankYou Points will be temporarily set as your payment method at checkout. To redeem this offer, you must pay for at least a portion of your order with Citi ThankYou Points.
$15 of savings will be reflected on the final order checkout page if the items in your order are eligible for the promotion credit.
I successfully purchased a $50 gift card with $35 payment.
Not sure when the offer will expire - "This promotion starts at 12:00am PST on August 1, 2019 and ends on the earlier of (i) at 11:59pm PST on 12/31/2019, or (ii) when 31,096 offers have been redeemed, or (iii) when Amazon elects to end the promotion"