Perhaps a "hint" was given within the article pasted in the link below?
A senior aircraft-leasing company executive familiar with Amazon’s plans said the company has approached several cargo-aircraft lessors to line up the planes. The company has had talks with Air Transport Services Group (ATSG), Atlas Air and Kalitta Air, according to sources, though Kalitta Chief Executive Connie Kalitta denied he has talked with Amazon.
Because Amazon doesn’t have an Air Operator’s Certificate, which it needs to fly airplanes commercially, it would have to turn to cargo-jet lessors to launch the business.
Amazon in talks to lease Boeing jets to launch its own air-cargo business | The Seattle Times