Add percentage based fee to WooCommerce cart

endo-handling-fee.php

<?php 
add_action( 'woocommerce_cart_calculate_fees','endo_handling_fee' );
function endo_handling_fee() {
     global $woocommerce;
 
     if ( is_admin() && ! defined( 'DOING_AJAX' ) )
          return;

     $subtotal = $woocommerce->cart->subtotal;
     $fee_percentage = 0.1; // 10 percent

     $fee = number_format( round( $subtotal * $fee_percentage, 2 ), 2 ); // rounded to two decimal places to match currency formatting

     $woocommerce->cart->add_fee( 'Handling', $fee, true, 'standard' );

}