Amazon is mainly a name brand.
They invite company's to list their products, in an Amazon store front and take a decent percentage off the top of the sale.
Amazon has their stuff together, but they are only a name brand selling a service, much like UPS.
A large percent of Amazon sales are with small companies that chose their own shipping method.
The only reason I know this is because my wife works for one, and controls what gets shipped,by whom.
This small company my wife works for does about $2 to $3 million in sales a year through Amazon. The bulk of it goes USPS,then UPS.FedEx gets zilch.
If you see a pkg that is wrapped as Amazon, that means they control the shipping method and are under contract to a specific carrier.
This is a huge number, but, it is only a percentile of the true volume that flows under their moniker.
They cannot control shipping methods within their "stores".