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. Their contract to deliver Amazon merchandise was cancelled the end of August 2019 because of delays.