We are implementing a solution that uses AWS ELB with auto-scaling.
The issue we are having is that sometimes 3rd parties that we integrate with, for example, payment gateways, require to "whitelist" our IPs. This is difficult to achieve with our setup since the IP of an auto-scaled instance is unpredictable.
I would like to refrain from using something like a separate "NAT instance", which in my mind would take all of the beauty and redundancy of our environment and create a single point of failure.
What is the best practice in this situation? Is there any suggested solution which allows for scalability and inherent reliability of EC2 and the associated services, but still ties all outbound connections to a limited set of IP addresses?