The report estimated that if the agency increased its parcel rates, Amazon’s shipping costs would rise by about $2.6 billion. Some of that money would presumably go to the Postal Service, which in 2014 handled 40 percent of Amazon’s packages, according to one 2015 estimate.

The report also suggested that Amazon’s high shipment volume gives it a greater opportunity to negotiate rates with shippers. Amazon has acknowledged in the past that it has an arrangement with the Postal Service, but it has rejected the idea that its shipments are being subsidized by the agency.

In a 2017 statement, Amazon said that its partnership with the Postal Service was “reviewed annually by the Postal Regulatory Commission,” which found its contracts to be profitable.