Setting woocommerce customer note through functions.php not working

In Woocommerce I’m trying to modify the Customer Order note to say “Ship with this provider” when a customer billing/shipping address is a specific city.

WordPress 5.2.2 WooCommerce 3.6.5

I’m using the woocommerce_thankyou hook, getting the order data via order ID and get_customer_note(), set_customer_note().

add_action('woocommerce_thankyou','route_mail_on_customer_location', 30, 1);

function route_mail_on_customer_location($order_id){
  $CP_cities = ["City 1", "City 2", "City 3"];

  $order = wc_get_order( $order_id );
  $curr_note = $order->get_customer_note();
  echo "<p>Customer note: " . $curr_note . "</p>";
  if((strpos($curr_note, "Ship with Canada Post.") == false) && (in_array($order->get_shipping_city(), $CP_cities, true))){
    $note = __($curr_note . ". Ship with Canada Post.");
    $order->set_customer_note($note);
  }
  echo "<p>Customer note: " . $order->get_customer_note() . "</p>";
}

The echo results display correctly.

Customer note: Test Note

Customer note: Test Note Ship with Canada Post.

When I check the order page the order note is only the original customer note.

Customer provided note:
Test Note

It looks like the setter isn’t sending the changes to the database. Is there a method I need to call to make sure that my changes are added to the DB, or should I just do it directly via a wpdb query?

EDIT: Corrected echo results to reflect code.

Answer

Just add the follows code snippet to achieve to task –

function modify_woocommerce_checkout_posted_data( $posted_data ){

    $CP_cities = array( 'city1', 'city2', 'city3' ); // make sure to replace with proper city data

    $curr_note = $posted_data['order_comments'];
    if( strpos($curr_note, 'Ship with Canada Post.') == false  && in_array( $posted_data['shipping_city'], $CP_cities ) ){
        $note = $curr_note . __(' Ship with Canada Post.', 'textdomain' );
        $posted_data['order_comments'] = $note;
    }
    return $posted_data;
}
add_filter( 'woocommerce_checkout_posted_data', 'modify_woocommerce_checkout_posted_data', 99 );