add_action( 'woocommerce_cart_calculate_fees','cod_extra' );
function cod_extra() {
    global $woocommerce;

    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

        $chosen_gateway = WC()->session->chosen_payment_method;
        $fee = 10; //varsayılan para birimiyle: $10, €10 gibi...
        if ( $chosen_gateway == 'cod' ) { 
            WC()->cart->add_fee( 'Kapıda ödeme ücreti', $fee, false, '' );
        }
}