I tried that on the main page, the mid page and the confirmation page...and ran all three pages thru builtwith - didnt spot anything obvious. The only thing that looks like it could be a possible answer is AppNexus under Advertising on builtwith. There are the usual suspects like analytics, retargetting and facebook. But I know for certain this site generates 100% sales via affiliate at a large scale.
When I look at AppNexus, I dont see that they do affiliate marketing (although I may be wrong on that part).
Any add'l ideas?