You are putting the value into the wrong field. The signature for qa_post_create is:
function qa_post_create($type, $parentid, $title, $content, $format='', $categoryid=null, $tags=null, $userid=null, $notify=null, $email=null, $extravalue=null, $name=null)
So you are putting the extravalue into the $notify value. You'll need to add some extra nulls, i.e.
qa_post_create($type, $parentid, $title, $content, $format, $categoryid, $tags, $userid, null, null, $extravalue)