A web company is looking to implement an external payment service into their highly available application deployed in a VPC Their application EC2 instances are ...

A web company is looking to implement an external payment service into their highly available application deployed in a VPC Their application EC2 instances are behind a public facing ELB. Auto scaling is used to add additional instances as traffic increases under normal load the application runs 2 instances in the AutoScaling group but at peak it can scale 3x in size. The application instances need to communicate with the payment service over the Internet which requires whitelisting of all public IP addresses used to communicate with it. A maximum of 4 whitelisting IP addresses are allowed at a time and can be added through anAPI.
How should they architect their solution?