Am I correct in thinking that the last mile delivery solution from Amazon strictly relies on independent contractors or do they have corporate trucks/drivers?
IIRC, it's all independent contractors. Whether it's an independent company contracted by Amazon (which is the bulk of it), or a direct Lyft/Uber-style gig economy model of sourcing contract drivers.
Currently both Amazon and FedEx both use independent contractors (sorry, entrepreneurs) - and your experience will vary widely with the contractors operating in your area. We always groan when FedEx is doing the delivery, because in our area that usually means two or three days of 'missed deliveries'.
They have at least some corporate trucks/drivers in at least some locations. Though without inside data it's impossible to say what proportion, but anecdotally it seems to be mostly independent contractors in Seattle.
Here's a (somewhat sensationalized, it's Buzzfeed News) article about some of the negatives that have shown up. It references that all of the delivery network is decentralized: https://www.buzzfeednews.com/article/carolineodonovan/amazon...