The full retail price in the US is $49.95. If you follow the normal activation path, the price is $49.95 (or whatever the retail price is for your locale)
To take advantage of the special offer, you needed to use a special link.
I recommend that you contact support to see if you can get a refund for the difference or cancel the purchase and be directed to the link for the special offer.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve