Add custom fee to cart automatically

functions.php

<?php
/**
 * WooCommerce Extra Feature
 *
 * Add custom fee to cart automatically
 *
 */
function woo_add_cart_fee() {
 
  global $woocommerce;
	
  $woocommerce->cart->add_fee( __('Custom', 'woocommerce'), 5 );
	
}