Sanders said that the Government Accountability Office (GAO) has ruled that the Postal Service cannot change delivery without congressional approval. Sanders also pointed to the 2013 continued spending resolution, which prohibited the USPS from stopping Saturday deliveries.

Earlier this year, Donahoe announced that in order to save money, USPS would eliminate Saturday mail service starting Aug. 1.

With a national debt of more than $16 trillion, the USPS has come under criticism from some lawmakers because it’s losing billions of dollars a year. During debate on the continued spending resolution last week, Sen. John McCainJohn Sidney McCainTo woo black voters in Georgia, Dems need to change their course of action Senate panel again looks to force Trump’s hand on cyber warfare strategy Senate panel advances 6B defense policy bill MORE (R-Ariz.) suggested that Congress should not stand in the way of the USPS cutting costs.

But Sanders blamed USPS’s financial woes on a mandate to prefund 75 years of future retiree health benefits over a 10-year period.

“No other government agency, no other corporation in America is burdened with this mandate,” the letter stated. “This pre-funding mandate is responsible for about 80 percent of the Postal Service’s financial losses since 2007.”

Sanders also blamed a legal restriction on USPS from offering “new and innovative” products and services.

“The other major reason that the USPS is financially strapped is because of a 2006 law banning the Postal Service from offering new and innovative products and services,” Sanders wrote. “It is clear to me and to many others that there are significant opportunities for increased revenue for the USPS, if given the opportunity to aggressively compete in the marketplace.”

Sanders said he looked forward to working with Donahoe to find solutions on making USPS more profitable.