Amazon gives non-Prime members free shipping at $35 or more of eligible items. Instead of simply letting users get the product with free shipping, they’ve added a discount that prices it exactly one cent below the $35 limit, while only subsidizing the price with $3.38, which is about half of what they’ll then charge you for shipping.
I don’t know if this is possible on Amazon, but on our local alternative (bol.com for the Dutchies) it is possible to buy e-books for one cent, which will get you free shipping. I have quite a selection of e-books I’ll probably never read, but which were just the right amount to get free shipping.