Best I understand it, and someone else can fill in the details if I misspeak, but you upload it with a price to Amazon. Then upload it to somewhere else, like Smashwords or another ebook retailer, and set the price to zero on there. Then, have some friends report this "better price" to Amazon. The Zon will then say "we can't have that!" and they price match the other retailer and make it permafree.