zasilkovna_hide_for_big_package

<?php 
add_filter( 'zasilkovna_shipping_cost', 'hide_zasilkovna_for_big_package' );
function hide_zasilkovna_for_big_package( $cost ){

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

    foreach($items as $item => $values) { 
      $_product = wc_get_product( $values['data']->get_id() ); 
      $lenght = $_product->get_length();
      if( $lenght > 50 ){
        $cost = false;
      }
    }
    return $cost;
}