You can blame Donald Trump for some things but the United States Postal Service (USPS) problem is not one of them.
First: Donald Trump is correct about voting by mail in the states that don’t have it already established. Many states with vote-by-mail still have problems. Three months is not enough. California is a good example where they threw away over 100,000 returned ballots because of irregularities with the ballot. Washington has seen its own in 2004, when King County “found” ballots in the “back room.”
Second: the USPS fiscal problems are not “voting by mail” issues.
As far back as 2009, the U.S. Government Accountability Office has been warning Congress, the President and the nation; “USPS must to fix their fiscal house.”
GAO says on their website that: “U.S. Postal Service’s Financial Viability — High Risk Issue”: