Back when I first started with eCommerce I had all of my stores hosted on the Yahoo Stores platform. They did not have a simple way to submit my feeds to Google Shopping (or any other major shopping comparison engines) so I started working with a company called AtenSoftware who managed all of my feeds for me. I learned A LOT of from the owner and he definitely knows what he's talking about when it comes to comparison shopping engines (and Google Shopping).
He just posted this free presentation on how to optimize your Google Shopping campaigns and I HIGHLY recommend it.
You can download the slideshow and read along with the presentation notes here:
Google Shopping Strategies Presentation
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