CSS means Comparison Shopping Service. It's an external comparison tool visible in Google Shopping which shows which companies offer the same product.
Why should I use a CSS?
If you choose an external CSS the 20% margin markup that Google's CSS invoices is completely deducted, which essentially means you bid 20% higher with the same CPC. In that case you will win more auctions than any competitor who is not using an external CSS without paying more.
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