Sitefinity - add product with varying price

//Price is good, use it!
OrdersManager manager = OrdersManager.GetManager();
CatalogManager cmanager = CatalogManager.GetManager();

if (CustomConfig.candleProductId == Guid.Empty) {
    ShowErrorMessage("A system error has occurred, please try again later.");
    return;
}

Product product = cmanager.GetProduct(CustomConfig.candleProductId);
if (product == null) {
    ShowErrorMessage("A system error has occurred, please try again later.");
    return;
}

int quantity = 1;
product.Price = amount;

CartOrder order = DeleteCartForUser(manager);
OptionsDetails optionsDetails = new OptionsDetails();

//Add the Carols message
product.Sku = txtMessage.Text;

manager.AddToCart(order, product, optionsDetails, quantity);

var checkoutPageUrl = SitefinityFunctions.GetPageUrl(WebsilkConfig.SitemapConfig.GetPageId("CheckoutPage"));
HttpContext.Current.Response.Redirect(checkoutPageUrl);