Offer code for Petzold Cosumer Preview eBook before 31May2012

I tried to buy the Programming Windows 6e, Consumer Preview eBook, by
Charles Petzold, but when I got to the Checkout the shopping cart was showing
$50 full price, not the $10 offer mentioned on
which I thought was valid until 31st May.
Is there a specific offer code I need to enter to get the $10 offer?
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