No, this is definitely something new. It used to be that anything that came from Amazon's warehouses could ship for free using Prime. Seaking, you are thinking of items sold by 3rd party sellers. The question is, did Amazon add new items to their warehouse in order to offer them with this new "add-on program," or did they take items that previously used to ship for free, and designate them as "add-on items?" Amazon is saying the former but it looks to me like some of the latter, too. And who's to say how many items they might change from Prime shipping to "add-on" in the future?
http://allthingsd.com/20120525/didnt...ands-of-items/
http://deals.woot.com/questions/deta...add-on-program
eP