Shipping cost not applied for vistors in prestashop 1.6

[Obada8]

My problem is when i'm non-customer on my store and do add to cart for orders the cart block show free shipping cost, but if i'm customer and i registered the shipping cost is applied from getPackageShippingCost function which located in /classes/Cart.php

I tried to make getPackageShippingCost to return only 10, but no luck.

This is my edit for this function:

    public function getPackageShippingCost($id_carrier = null, $use_tax = true, Country $default_country = null, $product_list = null, $id_zone = null)
    {
        if ($this->isVirtualCart())
            return 0;

            $order_total = $this->getOrderTotal(true, Cart::ONLY_PHYSICAL_PRODUCTS_WITHOUT_SHIPPING, $product_list);

        if ($order_total >= 30)
            return 0;
        else
            return 10;
    }

This work for customers but not work for visitors, it show free shipping for visitors.