We need to calculate the shipping charge without tax in some condition on WordPress WooCommerce.

Our rule was if user order amount < 50 then shipping charge would be $5 + tax 12%. Its runs fine.
But when we attempt to fire the rule that if user order amount > 50 then shipping charge would be -$5(-) but WooCommerce automatically added the tax -12%(-) with the shipping charge. We could not remove the tax -12%.

Here is our code:

$item = new WC_Order_Item_Shipping();
$item->set_props(
    array(
        'method_title' => "test",
        'method_id'    => "test",
        'cost'         => -5,
        'taxes'    => array(),
        'tax'    => array(),
        'calc_tax'  => 'per_order',
        'total'        => -5,
        'tax_class'    => NULL,
        'total_tax'    => 0,
        'tax_status'   => 'none',
        'meta_data' => array(),
        'package'   => false,
    )
);
$order->add_item( $item );
$order->calculate_totals();
$order->save();

Please advice us.