Shopsite nicely handles basic feeds to Google Merchant Search. But I understand that it doesn't take feeds of "Custom Product Attributes" that some merchants would find useful. I wouldn't think it would be difficult to include that capability. My observation is that G Merchant Search is rapidly growing.
Until that feature is added, the feed instructions should mention the omission.
see also http://support.shopsite.com/forums/viewtopic.php?f=1&t=8271
Thanks!