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