If you let people pay to be featured, you'll always end up favouring those who pay. And if you didn't favour those who pay, why would people pay in the first place?
This is the big problem with all recommendation sites: The easiest way to monetize them is by charging vendors for visibility; so sooner or later all recommendation sites start recommending the most profitable products. High quality fair priced products don't have a chance, since they will always be outbid by someone who makes a cheaper product or charges more.
I think the author implemented a tasteful and respectful solution. Other sites inject sponsors atop search results, every nth result, block parts of pages, force extra navs to link-spammed pages to get the product site, or sends sponsors to the top of rankings regardless of votes. You never have to glance at that portion if you're not interested, and can find it if you are. It's good design and advertises to the user in a respectful and convenient way imo. This is a small fixture that's static, and not even in the common paths followed by users ("F" eye track)
@Author: Perhaps a voting freeze during the sponsored period or a vote expiration (votes older than a certain amount of time fall off the total or such) would address the visibility = rank boost concern. Two random examples for moderating it with zero future thought there, but I think you get the idea.
If you want to make a "fair" recommendation site, you need to find some other way to monetize. One possibility would be to charge potential customers. I believe there are people out there who want fair recommendations, I'm just have no idea how to get them to pay.