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However, some of the products aren't matched properly (and I looked at some of my competitors, and theirs are even worse - lucky for me!)
So I got an email from the AdWords Blog announcing these were available to everyone, and it says something about controlling which products display with which ads by "making a simple addition to your Merchant Account" - but they don't say what the addition is! Where is it? Is it in the feed somewhere? If it's in the account, where is it? All I can find is that the two accounts are tied, not that I can specify products to an ad or ads.
I haven't even been able to see yet which products are being matched; the product extensions are not showing in my location, I guess.
Growing pains ...
ADDED: so far the only thing my merchant and I have been able to figure out is that we could make some edits then upload a new version of the feed.