It looks like Amazon has quietly raised the minimum purchase cost you have to pay in order to qualify for free shipping. The cost used to be $35, but has just raised another $14, bringing the total for free shipping up to $49.
The official wording on Amazon’s Free Shipping page reads,
“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 any 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.”
If you are an Amazon Prime member then you won’t have to worry about this, as unlimited free two-day shipping is part of your membership. This only affects those of you who don’t have a membership, possibly in an effort to get more people to sign up.
For further clarification on anything you can head on over to the Amazon Free Shipping page.