protected void Page_Load(object sender, EventArgs e)
{
    string rawId = Request.QueryString["ProductID"];
    int productId;
    if (!String.IsNullOrEmpty(rawId) && int.TryParse(rawId, out productId))
    {
        using (ShoppingCartActions usersShoppingCart = new ShoppingCartActions())
        {
            usersShoppingCart.AddToCart(Convert.ToInt16(rawId));
        }
    }
    else
    {
        ...
    }
}