public void AddItem(Item item)
    {
int countItem = Item.Count;
        if (countItem > 0)
        {
            for(int u = 0; u < countItem; u++)
            {
                if (Item[u].ID == item.ID)
                {
                    Item[u].Size += item.Size;
                }
                else
                {
                    Item.Add(item);
                }
            }
        }
        else
        {
            Item.Add(item);
        }
    }