如何在不同页面中使用计算值

I have made a shopping cart , but I am really struggling to get the the value of a calculation in a different page.

I have the following calculation in one page:

private function update_delivery() {


        if($this->subtotal > 329) {

                $this->delivery = 0;}

            if($this->subtotal < 329) { 
                $this->delivery = 7.99;
                }}

class Jcart {

    public $config     = array();
    private $items     = array();
    private $names     = array();
    private $prices    = array();
    private $qtys      = array();
    public $delivery      = 0;
    private $subtotal  = 0;
    private $itemCount = 0;


    function __construct() {

        // Get $config array
        include_once('config-loader.php');
        $this->config = $config;
    }

I want to use this->delivery value in a different page. This might be really easy, however I am really struggling, any help welcome, or point me in the right direction to look up how to do it will be great.

You can try and store that value in a $_SESSION variable. This kind of variables exists in every page where you have session_start(). For instance: $_SESSION['delivery']=$this->delivery.