Pre Order Price Guarantee. Or whatever they call it.Price Adjusted?
Basically, when you preorder an item with Amazon, if they ever lower the price of the item after you preorder it, you automatically get that lower price without having to re-order. If the price goes back up you still keep the lower price too.
Wave 4 amiibos on Amazon UK were ~£13.50 or something when they were put up. Ness and Lucina dropped to ~£10.80 when they were put back up for preorder (the rest didn't get a price drop for some reason though, hopefully next round)