It is my belief that syndication of this type will almost always produce less qualified leads. The major ppc engines do the same, but offer a way to filter "bad sites". I've not heard of any way to do this for shopping sites.
Are you able to see from which sites the traffic is coming from or just that it's shopping.com partners? If you can see which site, maybe just create a list of them, the traffic they sent that is not from qualified visitors, and send it to shopping.com. I can't imagine they would listen to just one webmaster, but we have to start somewhere.