The AWS team has confirmed that the incident has been resolved. See https://health.aws.amazon.com/health/status for details

The AWS team is continuing to work toward full restoration of the use1-az4 data center. We are not seeing a large volume of platform issues, but will keep this incident open until AWS confirms full restoration.

A subset of orders may be experiencing slow or failed checkout due to an upstream AWS networking issue affecting one availability zone in us-east-1. Most orders are unaffected. AWS health status: https://health.aws.amazon.com/health/status. We're monitoring and will update as the upstream issue resolves.