I've had free one day shipping for a while... and now Amazon breaks my orders up in to a couple of seperate groups when I check out, so even if I have $60 in stuff I'm buying they don't give me the free same day delivery option because the groups don't go over $35.. Of course, one day delivery is available - I just have to pay the $5.99 fee.
I don't know if there's some weird fine print about it, but it just seems like some bullshit so they don't ever have to give me one day shipping unless I buy one item that costs over $35 all by itself.
It's because all items aren't together at the same warehouse. The warehouses are basically split into two styles. One that holds large items (tvs, consoles, outdoor equipment etc), and one that holds small (think CDs, games and smaller).
When someone orders both, the small items are shipped to the other warehouse with large items. They are then combined and shipped out. Now, some of the large item warehouses have some of the more popular small items, but usually not.
Also, the ones that have items that are traded in or returned are a crapchute on whether you'll get it through one day delivery, because those items are spread out all over the US.
Basically, you'll only get one day shipping free on items that are at the closest warehouse that allows it.