Anybody ever face a CORS error when posting to a Suitelet ? we are talking about posting from one Suitelet URL to another... which mean they are technically in the same host...

That is weird. Have you tried adding

response.addHeader({
                name: 'Access-Control-Allow-Origin',
                value: '*'
            });

to your suitelet?

NetSuite does not allow you to manipulate that.