Getting "The authorization grant type is not supported by the authorization server" from amazon
Change CURLOPT_HTTPHEADER from
Content-Type: application/x-www-form-urlencoded
to
Content-Type: application/json
Try to add 'token_type' => 'bearer' to your $postfields data.
Here is from Amazon developer documentation:"Access Token Request ... The type of token returned. Should be bearer."
"Access Token Response: ... unsupported_grant_type The client specified the wrong token_type."