Definitely sounds like an affiliate model. Especially if Adsensers are allowed to actually choose the PPA ads.
From the Inside AdSense blog:
Google can help make sure you find and display the most relevant, best-performing referral ads on your site. Simply provide a few keywords to describe your site and your users' interests, and you'll see ads automatically displayed that perform best for your site. No more constant updating, rotating products, or guesswork.