How do I purchase and start up a Reserved Instance?
You purchase an EC2 Reserved Instance by calling the PurchaseReservedInstancesOffering API method. Launching a Reserved Instance is no different than launching an On-Demand Instance. You simply use the RunInstances command or launch an instance via the AWS Management Console. Amazon EC2 will optimally apply the cheapest rate that you are eligible for in the background.
How do I control which instances are billed at the Reserved Instance rate?
The RunInstances command does not distinguish between On-Demand and Reserved Instances. When computing your bill, our system will automatically optimize which instances are charged at the lower Reserved Instance rate to ensure you always pay the lowest amount.