<?php 
* Auto Complete all WooCommerce orders.
*/
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 );
   $order->update_status( 'completed' );
}