S3 POST request to S3 with response 405

This isn't CORS related -- it's S3 itself. The S3 website endpoints are only equipped for GET and HEAD. Anything else should be denied before the redirection rules are checked.

Website Endpoint

Supports only GET and HEAD requests on objects.

— http://docs.aws.amazon.com/AmazonS3/latest/dev/WebsiteEndpoints.html