You may want to do the IFRAME approach the other way around -- specify a common thank-you page URL in Seller Admin > Edit Account Preferences, then place an IFRAME inside that thank-you page on your site, with src=https://www.e-junkie.com/ecom/rp.php?noredirect=true&txnid=XXXXX -- that XXXXX would be the transaction ID we appended to your thank-you page URL, so your page just needs some JavaScript (or PHP or other scripting) to read its own URL, grab the txn_id value from that, and append that value into the IFRAME src= URL.
BTW, be advised that not all buyers necessarily reach the thank-you page. PayPal Pro is the only payment processor that automatically refreshes to your thank-you page after successful checkout; regular PayPal standard and all other processors only provide a return/continue/finish link on their checkout-completed page, which link the buyer must choose to click manually in order to reach any thank-you page.
One way around this would be to enable the "Send payment data to a URL" setting for each product, then specify the URL of a script at your end. Once we receive notification from the processor that the buyer's payment was good and completed, we would POST order data to your specified URL, where your scripting could then invoke your Google AdWords/Analytics/Optimizer code automatically.
FWIW, for any checkouts done via Google Checkout, our integration with Google Analytics will report the sale conversion to Analytics even if the buyer does not click-through to the thank-you page, so Google may provide some way to integrate Analytics with Optimizer and take advantage of that, if you don't mind using Google Checkout as your sole payment processor.