One year ago Amazon raised the total order price of free shipping from $35 to $49. It seems as though the company has reversed this decision a few days ago and you once again have to pay the old price from all product categories. Orders including $25 or more of eligible books also qualify for FREE Shipping. With free shipping, your order will be delivered 5–8 business days after all your items are available to ship, including pre-order items.
Amazon was likely pressured into lowering their minimum order price because of Walmart. The big box retailer introduced a new policy a few weeks ago that provides free shipping on orders of $35 or more.