I don't want Elastic IP to be enabled for these apps as they are short lived.
Se você estiver em uma sub-rede pública de VPC (o que é provável), será necessário que uma instância se comunique com a Internet. Por link :
To enable an instance in your public subnet to communicate with the Internet, it must have a public IP address or an Elastic IP address that's associated with a private IP address on your instance. Your instance is only aware of the private (internal) IP address space defined within the VPC and subnet. The Internet gateway logically provides the one-to-one NAT on behalf of your instance, so that when traffic leaves your VPC subnet and goes to the Internet, the reply address field is set to the public IP address or Elastic IP address of your instance, and not its private IP address. Conversely, traffic that's destined for public IP address or Elastic IP address of your instance has its destination address translated into the instance's private IP address before the traffic is delivered to the VPC.
Os EIPs atribuídos automaticamente que vêm com uma nova instância não custam nada e desaparecem automaticamente quando a instância é desatribuída.