That is what I was thinking and then I realized how UPS offloads delivery to USPS. It could be because they are overloaded.

Further, there is too much risk and loss of power when UPS's biggest customer is just one company. UPS probably doesn't want that risk and it would rather stay moderately sized than put all of its eggs in one basket.

But more importantly, it is Amazon that is taking these steps and I predict, because they have been hiring large swaths of people that many years ago they would not have even thought of, Amazon is beginning to think about doing business over the course of the next hundred years. It is preparing for those hundred years now, knowing that airplanes will be integral to continue meeting demand. These airplanes will most likely run side by side with their partner delivery companies.