Amazon EC2 Reserved Instances and Other AWS Service Reservation Models
AWS Whitepaper

Amazon DynamoDB Reservations

If you can predict your need for Amazon DynamoDB read-and-write throughput, reserved capacity offers significant savings over the normal price of DynamoDB provisioned throughput capacity. You pay a one-time upfront fee and commit to paying for a minimum usage level at specific hourly rates for the duration of the reserved capacity term. Any throughput you provision in excess of your reserved capacity is billed at standard rates for provisioned throughput.