Autopostback resetting my list

Question

I want to store items to a list but everytime i press the confirm button to add one item to the list the page will refresh and reset my list. How do i stop this from happening whilst at the same time have an add button to keep adding to the list?

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
    }
    else
    {
        slist = (List<Shopping>)ViewState["Items"];
    }
}

List:

private static List<Shopping> slist;
        public List<Shopping> GetShopping()
        {
            return slist;
        } 

Button Click:

slist = new List<Shopping>(); 
            Shopping s = new Shopping();
            s.Item1 = txtItem1.Text;
            s.Item2 = txtItem2.Text;
            s.Item3 = txtItem3.Text;
            s.Item4 = txtItem3.Text;

            slist.Add(s);
            ViewState["Items"] = slist;
            showShopping();

Method:

showShopping()
{
GridView1.DataSource = GetShopping();
            GridView1.DataBind();
}

Answer 1

Do not store business objects in ViewState. Use the Session

private List<Shopping> ShoppingList
{
   get { 
          var shopping = Session["Shopping"] as List<Shopping>;
          if (shopping == null)
          {
              shopping = new List<Shopping>();
              Session["Shopping"] = shopping;
          }
          return shopping;
       }
   set { Session["Shopping"] = value; }
}

Button handler

var shopping = ShoppingList;
shopping.Add(new Shopping());
...