Cloudfront Flat-Rate - what comes after Premium?

For workloads that exceed the Premium tier limits (500 million requests and 50TB per month per distribution), AWS offers Private Pricing Agreements (PPAs). These are custom pricing arrangements that you negotiate directly with AWS based on your committed usage volumes.

PPAs are usage-based rather than fixed-fee, meaning you pay for what you consume at your negotiated rate. They're specifically designed for high-volume customers and typically provide the best economics at scale. An important benefit is that PPA customers can still purchase Flat-Rate Plans for individual distributions if needed, and that usage won't count toward their PPA commitment - giving you flexibility to mix pricing models across different distributions.

Given that your application exceeds the 500 million request threshold, a Private Pricing Agreement would be the appropriate next step. Since you've already reached out to the sales team, I'd recommend following up on that inquiry, as PPAs require direct negotiation with AWS to establish custom rates based on your specific usage patterns and volume commitments.
Sources
CloudFront Pricing Models: Flat Rate, Pay-As-You-Go, and Private Pricing — Which One Is Right for You? | AWS re:Post