What are shopping comparison sites doing about click-fraud

Hi Everyone,
Most shopping sites charge between $0.10 to $1.00 a click with a minimum deposit of $100 (except pronto is only $50).
We all know where there is CPC the is Click-Fraud! So what are these sites doing to combat it?
Major sites
Sites:
BizRate.com
shopping.yahoo.com
pronto.com
shopping.com
smarter.com
Sites immune to click-fraud
google.com/products (the old froogle)
jellyfish.com (CPA)
Thanks for your insite!

Original post by nytrokiss