They can always find another job more to their liking and skill level - nothing keeping them an Amazon. If their current skill levels are not adequate to find a higher paying job, additional school/training is an option. Seems like Amazon has a lot of resume's on file and no problem attracting people willing to work.
What does the pay rate for Amazon employees have to do with UPS stock?
How about UPS & FDX provide a better service and then Amazon does not have to resort to doing it themselves? If they were meeting Amazon's needs, Amazon would have no reason to look elsewhere. Competition is a good thing and it pushes companies to perform better. If they don't perform better, they lose customers. Are you opposed to the free market and competition?
If you are a former UPS employee you should know about My Choice and should use it. No more waiting around for packages. You can even electronically sign for shipments and tell the driver where to leave it. Why would anybody wait at home for a package when there is no need to?