Amazon often uses smaller regional delivery companies. I assume they're cheaper for the areas they serve. In the DC area, a lot of Amazon orders are delivered with Lasership, which pretty much fits your description. They drive shoddy white cargo vans with PACKAGE DELIVERY slapped haphazardly on the side. Their deliveries often don't make it. Once they delivered my package 30 miles away for no apparent reason (completely unrelated family name, no similarities in address, etc.). Amazon's use of them seems to have declined recently, but they're still using them and it really doesn't seem like a good idea.

Amazon does a bunch of drop ship orders. If a vendor gets a local order, I can imagine they may decide to save on shipping by sending an employee to hand deliver the product. Or it could have been a local courier service (same day delivery uses couriers).

The package I had delivered today came via Amazon Fresh - the grocery delivery service they are testing in the Seattle area. It looks like they're also using it for non-Fresh items too for small, commodity items like CDs

This is Amazon owning the entire process from order until the item is in my hand.

I use Amazon fresh all the time, and it's possible to get all sorts of non-grocery things through it. I think it's whatever Amazon.com stuff they have available in the local warehouse.