Reading the title, I thought they might be doing something genuinely interesting, by subsidising the cost of the car so long as you have a space in it which Amazon had access to which can be used for ferrying goods from X to Y based on your normal travel patterns. i.e. using personal vehicles as deadhead capacity, effectively.
Edit: Further thought. What if they're about to deliver to your car, having found it from the GPS beacon, and you decide to drive somewhere. Are they going to chase you down the autobahn, waving frantically at you?! How will they manage delivery schedules?
I can't help but think that this is a logistics nightmare.
Edit: Further further thought. What if they can't fit it in your trunk? What if you park backed up to a wall?