<?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;
    }
  }
}