"If HR 8015 were presented to the President, his advisers would recommend that he veto the bill", the agency said in the letter on Friday.
The White House said it opposes the bill because it claims it would place counterproductive restrictions on the US Postal Service's limited operational flexibilities and would do nothing to improve the agency.
On Saturday, the House of Representatives will vote on a bill to provide $25 billion in support to the US Postal Service.
US Postmaster General Louis DeJoy has implemented controversial changes that have caused delays ahead of the November vote. However, under criticism by Democrats, DeJoy announced a temporary pause in operational changes in the postal service.
Trump has said that the US Postal Service has been failing for decades and the planned reforms were meant to save it billions of dollars. Trump also said that mail-in ballots pose a risk of widespread fraud, citing the botched New York congressional primary race as an example. Trump’s opponents say they believe the changes are delaying the mail and threatening to disenfranchise voters.