WooCommerce - Keep product prices fixed on manual orders

/**
 * Ensures that WooCommerce deducts taxes applicable order's billing or shipping address
 * when adding items to manual orders.
 *
 * HOW TO USE THIS CODE
 * Simply add the code to the bottom of your theme's functions.php file, and it
 * will run automatically. For more information: https://www.skyverge.com/blog/add-custom-code-to-wordpress/
 *
 * GPL DISCLAIMER
 * Because this code program is free of charge, there is no warranty for it, to the extent permitted by applicable law.
 * Except when otherwise stated in writing the copyright holders and/or other parties provide the program "as is"
 * without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of
 * merchantability and fitness for a particular purpose. The entire risk as to the quality and performance of the program
 * is with you. should the program prove defective, you assume the cost of all necessary servicing, repair or correction.
 *
 * Need a consultation, or assistance to customise this code? Find us on Codeable: https://aelia.co/hire_us
 *
 * @return string
 */

add_action('wp_ajax_woocommerce_add_order_item', 'set_customer_address_for_add_order_item', 5);
add_action('wp_ajax_nopriv_woocommerce_add_order_item', 'set_customer_address_for_add_order_item', 5);

/**
 * Intercepts the "add order item" Ajax call, to override the billing and shipping
 * address on the customer entity.
 */
function set_customer_address_for_add_order_item() {
  if(!check_ajax_referer( 'order-item', 'security', false) || !current_user_can('edit_shop_orders')) {
    return;
  }

  if(!isset($_POST['order_id'])) {
    return;
  }
  $order = wc_get_order($_POST['order_id']);
  // Set the billing address against the customer entity. This will ensure that WooCommerce
  // will deduct the taxes applicable to that address, rather than shop's base address
  WC()->customer->set_billing_country($order->get_billing_country());
  WC()->customer->set_billing_state($order->get_billing_state());
  WC()->customer->set_billing_postcode($order->get_billing_postcode());
  WC()->customer->set_billing_city($order->get_billing_city());

  // Set the billing address against the customer entity. This will ensure that WooCommerce
  // will deduct the taxes applicable to that address, rather than shop's base address
  WC()->customer->set_shipping_country(!empty($order->get_shipping_country()) ? $order->get_shipping_country() : $order->get_billing_country());
  WC()->customer->set_shipping_state(!empty($order->get_shipping_state()) ? $order->get_shipping_state() : $order->get_billing_state());
  WC()->customer->set_shipping_postcode(!empty($order->get_shipping_postcode()) ? $order->get_shipping_postcode() : $order->get_billing_postcode());
  WC()->customer->set_shipping_city(!empty($order->get_shipping_city()) ? $order->get_shipping_city() : $order->get_billing_city());
}