Its a while since I have used eCart rules but I am pretty sure that you just trigger it based on a value.
You would set it to use a shipping value of 0 if a certain value is found. I think the trigger terminology is 'Based on a value'
And in the value field you would write
$_SESSION['freeshipping'] == '1'
Sorry for being a bit vague, but hopefully will point in the right direction a bit.