I don't know how this works in general but I assume that there are some heuristics as to whether shippers require delivery signatures or not. Because of where I live and how isolated my house is the only real dangers are misdelivery and damage.

One of the early-on Prime benefits for me was fairly deterministic delivery. At a time when I ordered lot of physical books, CDs, and DVDs, the minimum amount for free shipping wasn't a big deal. But random order arrivals when I was traveling much of the time were.