if ($_SESSION["sample_count"] == $_SESSION["cart_items_count"]){

    $sample_count = $_SESSION["sample_count"];
    $sample_shipping_cost = 4;
    $woocommerce->cart->add_fee( 'Samples Postage', ($sample_count * 4), true, '' );
    set_cart_contents_weight(0);
}

<td data-title="Transport Costs">
    <ul id="shipping_method">
        <li>
            <input type="radio" name="shipping_method[0]" data-index="0" id="shipping_method_0_wbs10065ab3a2_delivery" value="wbs:10:065ab3a2_delivery" class="shipping_method"  checked='checked' />
            <label for="shipping_method_0_wbs10065ab3a2_delivery">Delivery: <span class="woocommerce-Price-amount amount"><span class="woocommerce-Price-currencySymbol">&#36;</span>6.28</span></label>
        </li>
        <li>
            <input type="radio" name="shipping_method[0]" data-index="0" id="shipping_method_0_local_pickup16" value="local_pickup:16" class="shipping_method"  />
            <label for="shipping_method_0_local_pickup16">Local pickup</label>
        </li>
    </ul>

add_filter( 'woocommerce_cart_needs_shipping', 'show_hide_shipping_methods' );
function show_hide_shipping_methods( $needs_shipping ) {
    $cart_items_total = WC()->cart->get_cart_contents_total();

    if ( $cart_items_total < 40 ) {
        $needs_shipping = false;

        // Optional: Enable shipping address form
        add_filter('woocommerce_cart_needs_shipping_address', '__return_true' );
    }

    return $needs_shipping;
}