// Adds current product to cart; gets shopping cart model from session and updates (then writes back to session).
        protected void btnAddToCart_Click(object sender, EventArgs e)
        {
            if (IsValid)
            {
                BLProduct productData = Session["Product"] as BLProduct;

                BLShoppingCart cart = HttpContext.Current.Session["Cart"] as BLShoppingCart;

                bool existingItem = false;

                foreach (BLCartItem item in cart.Items)
                {
                    if (item.Product.prodNumber == productData.prodNumber &&
                        item.Size == rblSizeOption.SelectedItem.Value)
                    {
                        item.Quantity += Convert.ToInt32(tbxQuantity.Text);
                        existingItem   = true;
                        break;
                    }
                }

                if (!existingItem)
                {
                    cart.AddCartItem(new BLCartItem(productData, rblSizeOption.SelectedItem.Value, Convert.ToInt32(tbxQuantity.Text)));
                }

                Response.Redirect("~/UL/Cart");
            }
        }