A City Carrier Assistant to the United States Postal Service, unloads her mail truck after collecting mail on the busiest mailing day of the year for the U.S. Postal Service in Miami, Florida.

The United States Postal Service should have more flexibility to raise rates and should have the authority to charge market rates to deliver most packages, according to a task force created following criticism by President Donald Trump that it provides too much service to Amazon.com for too little money.

The task force, which was created in April, was asked to make recommendations on how the U.S. Postal Service, an independent agency within the federal government, could address financial losses. The postal service lost almost $4 billion in fiscal 2018, which ended on Sept. 30, even as package deliveries rose.

Trump has repeatedly attacked Amazon on Twitter for treating the Postal Service as its "delivery boy" by paying less than it should for deliveries and contributing to the service's $65 billion loss since the global financial crisis of 2007 to 2009, without presenting evidence.

Amazon shares were down nearly 5 percent at $86.58 in afternoon trading on Tuesday.

CNBC's Anjali Robins contributed to this report.

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