Convinced they do this with my shipments too. I confirmed with them over chat this is the case.
I chose 5-8 free.
The package did not ship until day 7 and it arrived to me on day 8. First class mail that probably shipped from a facility near me. If they had just shipped it on day 0, I would have gotten it on day 1 (and it would cost them on their side the exact same amount of money) but of course that would cause their "Prime" service to lose usefulness.
All in all, Amazon does so many other things right that I could care less about their tactics in this matter. It was sweet in the early days of Newegg where "Saver" 7 days would get to me in two days, but alas, it's worth mentally preparing yourself that Amazon free shipping means "as late as possible without breaching contract".