One of the nice perks of some Citi reward credit cards is that you can use your Thank You points to pay for things you buy on Amazon.com. At the end of each billing cycle, your credit card will update the number of points available. Make sure to quickly use them, because they do expire. The Thank You Preferred credit card only gives you points when you purchase items; the Citi Double Cash gives also awards you ThankYou points when you pay your balance!
If needed, add the Citi card to your Amazon account: once logged in, mouse over the "Your Account" dropdown (top right corner) and pick "Your Account". Next, click "Add a Credit or Debit Card" in the second group of settings. Note that this isn't the only rewards card you can use on Amazon. Others include the Chase Ultimate Reward card, the Amazon.com Rewards Visa, the American Express Membership Rewards, and the Discover Cashback Bonus.
Now that the card is added, it will take a day or two to synchronize with your Citibank credit card and Amazon accounts. Once that's done, check out from your shopping cart page, just like you normally do. The last screen, right before you pay for your order, lets you choose your payment method. (Change your default credit card for Amazon payments to the Citi card, so it's automatic).
For whatever reason, your Citi ThankYou Rewards points don't always show up in the checkout screen. If that happens, click the link in the "Please refresh to see your rewards balance" message. That should update the available total.
To use all your ThankYou points to pay part of, or the entire order, click Apply. If you want to only use some of your points, update the dollar amount in the box to a lower sum, and then click on the Apply button. By default, any Amazon gift card balance will be applied first, and the rest will be charged to the default payment method. Using your reward points overrides that, so you can choose how to pay for your order. Enjoy your free monthly Amazon orders!