https://aws.amazon.com/about-aws/whats-new/2017/10/announcin... shows the four regions we are currently available in.

On https://aws.amazon.com/rds/aurora/pricing/, if you go to the "PostgreSQL Compatible" dropdown, you will see those same four regions.

We're working on making that page easier to parse, and the fact that you just ran into this confusion reinforces our committment to fix it - thank you!