automatically-complete-orders.php

/**
 * Auto Complete all WooCommerce orders.
 */
function wb_custom_woocommerce_auto_complete_order( $order_id ) {
	if ( ! $order_id ) {
		return;
	}

	$order = wc_get_order( $order_id );
	$order->update_status( 'completed' );
}
add_action( 'woocommerce_thankyou', 'wb_custom_woocommerce_auto_complete_order' );