There have been many unhappy customers of Amazon because of delayed delivery by USPS. Amazon’s pride is speed of delivery, and customers are frequently disappointed when USPS delivers 2-3 days after the promised date.
Sure, Amazon’s delivery fleet is growing. There are 70 airplanes, 40,000 semi-trailers, 30,000 vans and other delivery vehicles that service the U.S. UPS gets a portion of the merchandise to be delivered while FedEx gets none. Amazon and FedEx cut ties in August 2019.
Jason Boyce owns the Amazon Agency Avenue 7 Media, a seller management group, has many clients including Tormago and Wall in Tets which makes and sells dog wheelchairs. He told me of delivery problems. USPS is used for many small-town deliveries, but has failed to meet delivery dates.