You are forgetting about the fact that the truck, and the industry behind it, already exist.
How many new factories, mining operations, flights and ships will be required to start providing Amazon with the millions of new drones per year it will need to achieve this?
This is not just a quick and easy way to replace a 10 minute detour. It is a new multi billion dollar industry which uses countless tons of materials and more energy to create millions more new plastic 'things' which will cost lots in energy and materials whilst simultaneously causing lots more waste on the planet.