How/whether to use $this-> on a Class/Object pulled from a Session?

I'm currently refactoring a shopping Cart and I'm not sure whether I should be redeclaring it or using $this or maybe do something in the constructor or something when I pull it from the Session.

So e.g. my controller creates a new Cart, in order to call the 'add' method.

And then in the add method in the Cart I pull it from the session if it exists already.

Running the setTotals() method I pass the $cart object, but can I just use $this? Same happens in some other methods. Feel like I am creating extra Objects when I should be using the original.

Simplified version below, any ideas?

Controller

 //AJAX POST TO ADD PRODUCT TO CART
    public function postAddToCart(Request $request) {

        $cart = new Cart();
        $cart->add($request->id); 
        return response()->json(['status' => 'success', 'message'    => 'Item Added'], 200);

    }

And then Cart Class

class Cart {
    
    public $items = null; 
    public $itemsTotal= 0;
     
    
    // //GET CART = AJAX REQUEST
    static function get() {
        if(Session::has('cart')) { $cart = Session::get('cart'); } else { $cart = null;}
        return response()->json(['cart' => $cart,], 200);
    }
    
    //ADD A PRODUCT TO THE CART
    public function add($id) {
        $cart = session()->pull('cart', function() { return new Cart(); });
        $item = Panel::where('id',$id)->with('sub_panels')->with('upgrades')->first(); 
        $cart->items[$item->id] = $item;
        $cart->setTotals($cart);
    }

    private function setTotals($cart){
        
        $cart->itemsTotal = 0;
        $cart->customCharge = null;
        
        $cart->setItemsIntervalPricing();
        $cart->setCollectionPrice();
        $cart->applyDiscountToTotals();
       

        session()->put('cart', $cart);
        
    }