change_cost_zasilkovna_for_product.php

<?php

add_filter( 'zasilkovna_shipping_cost', 'change_cost_zasilkovna_for_product' );
function change_cost_zasilkovna_for_product( $cost ){

    $items = WC()->cart->get_cart();

    foreach($items as $item => $values) {
        if( $values['data']->get_id() == 31 ){
           $cost = $cost * 3;
        }
    }

    return $cost;

}