please use code tags when posting code.
I’m not sure exactly what you’re doing, but if you want to set the default value of a field then I think you need to use the acf/load_field filter. the acf/update_value runs when before the acf field is saved, not when the field is loaded.
http://www.advancedcustomfields.com/resources/acfload_field/
You can’t change the default value of an ACF field when you’re saving a product in woocommerce.
Also, looking at what I can make out of your code $order->id has no meaning inside of the function.
$post_id = $order->id; is setting $post_id to NULL.
