DynamoDB has provisioned throughput that a user can request i.e. the write capacity unit and the read capacity unit for both auto scaling and manual throughput provisioning. You are billed hourly for using the resource once you exceed the free-tier limit. You will be billed hourly for your throughput capacity whether you are sending requests or not.
You can check the rates for using DynamoDB here: https://aws.amazon.com/dynamodb/pricing/