Reserved Instances billing
All Reserved Instances provide you with a discount compared to On-Demand Instance pricing. With Reserved Instances, you pay for the entire term regardless of actual use. You can choose to pay for your Reserved Instance upfront, partially upfront, or monthly, depending on the payment option specified for the Reserved Instance.
When Reserved Instances expire, you are charged On-Demand Instance rates. You can queue a Reserved Instance for purchase up to three years in advance. This can help you ensure that you have uninterrupted coverage. For more information, see Queuing your purchase.
You can set up a billing alert to warn you when your bill exceeds a threshold that you define. For more information, see Monitoring Charges with Alerts and Notifications.