Autocomplete orders for a specific product in WooCommerce

auto-complete.php

<?php 
/**
* Auto Complete all WooCommerce orders of a specific product.
*/
add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_order' );
function custom_woocommerce_auto_complete_order( $order_id ) {
  if ( ! $order_id ) {
    return;
  }
  
  $order = wc_get_order( $order_id );
  
  foreach ( $order->get_items() as $item_id => $item_values ) {
    
    $product_id = $item_values->get_product_id();
    
    if ( $product_id == 326 ) {
      $order->update_status( 'completed' );
      return;
    }
  }
}