Sending Subdomain POST Data to Root Domain

Hi All, I was told that this would be a great resource to ask this question. We are building a web application where when a user creates an account, a subdomain is created (user.site.com). I need to be able to submit POST data from user.site.com to site.com but that is a violation of Same Origin Policy. What is the best way to go about this? I've looked a little bit into JSONP requests as well as Cross-Origin Resource Sharing (http://websitez.com/javascript-cross-domain-post-get/). Any help is greatly appreciated. Thanks, Scott