I wasn't aware of it either. Here's one of the sources:
Guess who just became Amazon's biggest shipper
The carrier would no longer deliver FedEx Express shipments in the United States. The shipper downplayed the move, pointing out that Amazon packages represented less than 1.3% of FedEx's total revenue in 2018.
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...Amazon is now its own biggest shipper....Amazon was delivering nearly half of its own packages by the end of May, according to Rakutan Intelligence, by way of a report in Axios. The e-commerce leader was responsible for 48% of its shipments, while the U.S. Postal Service delivered 33%, and United Parcel Service (NYSE: UPS) carried 17%. FedEx rounded out the list with less than 2%.
This represents a drastic change from just two years ago, when the Postal Service delivered about 63% and Amazon carried about 15% of its packages. At that time, UPS carried about 20%, while FedEx carried less than 2%, similar to its recent volume.