amazon ec2 ip whitelisted on a third party service provider
A possible approach could be:

Put your instances in a private subnet
Route incoming traffic through a load balancer
Route outgoing traffic through a NAT

Outgoing traffic will then use the elastic IP assigned to your NAT as its source IP.
An added benefit of this approach from a security perspective is that your instances won't be directly reachable from the internet, thus reducing your attack surface.