Unfortunately, those are still the best methods, even for the most current version of X-Cart. I would have thought they would implement it in new versions, but no such luck.
The script being referred to in that thread is cart.php.
You may also want to look at Jon's EZ Upsell mod, which handles the add to cart process quite nicely I think:
http://www.websitecm.com/store/customer/product.php?productid=84&cat=0&page=