First, please see this related reply:
http://www.e-junkie.com/bb/topic/3506/pg/0#post10697
Even if you can't use an iframe per-se for whatever reason, you'd still need a similar approach, where scripting in your site's thank-you page would grab the value for txn_id= (which we append to your URL during the redirection) and then rewrite that value into your page somewhere, perhaps in a link or a meta-refresh tag, where our thank-you page URL your linking or redirecting to would be:
https://www.e-junkie.com/ecom/rp.php?noredirect=true&txnid=XXXX
(where XXXX is the unique ID for each transaction).
I don't think an Apache rewrite rule would work, since our thank-you/download URL is unique for each transaction, and I'm not familiar with any way to dynamically write a query parameter value into an apache rewrite, but perhaps greater wizards than I may know a way...
All of this still leaves the problem where tracking won't work at all for buyers who have javascript and/or cookies disabled in their browser.