Adding a fee if chosen payment method is COD

mx-cod-fee.php

add_action( 'woocommerce_cart_calculate_fees', 'mx_add_cod_fee', 20, 1 );
function mx_add_cod_fee( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;
    $your_payment_id      = 'cod'; // Metoda plaćanja
    $fee_amount           = 6; // Naknada
    $chosen_payment_method_id  = WC()->session->get( 'chosen_payment_method' );

    if ( $chosen_payment_method_id == $your_payment_id ) {
        $fee_text = __( "Troškovi plaćanja pouzećem", "woocommerce" );
        $cart->add_fee( $fee_text, $fee_amount, false ); // Promijeniti u true ako trebaš obračun/prikaz poreza na naknadu
    }
}

add_action( 'wp_footer', 'refresh_checkout_script' );
function refresh_checkout_script() {
    if( is_checkout() && ! is_wc_endpoint_url('order-received') ) :
    ?>
    <script type="text/javascript">
    jQuery(function($){
        $('form.woocommerce-checkout').on( 'change', 'input[name="payment_method"]', function(){
            $('body').trigger('update_checkout');
        });
    })
    </script>
    <?php
    endif;
}