Amazon has quietly upped the price requirement you need to hit before you're able to take advantage of free shipping. That is if you're not a Prime member, of course. It used to be the case where you'd need to add $35 to your cart to get free shipping, but this has been increased to $49. Your order is also delivered at no charge if you have $25 or more worth of books in the cart.
"You receive free shipping if your order includes $25 of books (in which case all other eligible items in the order also ship free), or at least $49 of all other eligible items. Any item with 'FREE Shipping' messaging on the product detail page, that is fulfilled and shipped by Amazon, is eligible and contributes to your free shipping order minimum."
It's clearly a move to get more customers subscribing to Prime, forking out the subscription fee each month to take advantage of available services and unlimited two-day shipping. Should you be rocking Amazon Prime, you needn't worry. This only affects those who don't.
Thanks to Robbie P. for the tip!