There's another twist to Amazon's launch of the A9 search engine, blogged here yesterday. Amazon is discounting purchases by about 1.5 percent for A9 users—transparently, with no effort on the users' part, as long as they are signed in at A9 with their Amazon-registered identity.

"[User name], since you've been using A9.com recently, virtually everything at Amazon.com is automatically an additional p/2% (1.57%) off for you. Collecting this discount is zero effort on your part. It will be applied automatically at checkout (it will happen whether you use the shopping cart or our 1-Click Shopping®). You don't need to do anything to get this discount except keep using A9.com as your regular search engine.

"We don't advertise this additional discount that we give in exchange for using A9.com, so if you want your friends to know about it, please tell them. It is probably the only way they'll find out. All they have to do is use A9.com as their regular search engine. They should make sure they are signed in to A9.com (it should be recognizing them by name) so that we can be certain they get credit for their visit."

Amazon claims that it is sharing clickthrough revenue derived from Google AdWords displayed on A9 search results pages.