The solution from the forum actually works in your store. However, the tracking code was not displayed because of the templates cache. We've cleaned the cache up, and as far as we can see, now the Google Analytics, BazaarVoice and some other tracking code is included into the invoice page correctly. Please check it from your side.
You may find the information regarding the X-Cart templates cache here: http://help.x-cart.com/index.php?title=X-Cart:Advanced_Tools#Clear_Templates.2FX-Cart_Cache