For further reading on everything related to the U.S. Postal Service:
The Washington Post gives an overview of how policy and organizational changes at the U.S. Postal Service could lead to disenfranchisement.
BuzzFeed News explains the issues the Postal Service is facing right now, including how a 2006 law contributed to the agency’s financial problems. 
The Atlantic says restrictions and reforms introduced by the recently appointed postmaster general are the most significant threat to voting by mail.
FiveThirtyEight has a polling roundup that encapsulates how Americans feel about voting by mail.
The Associated Press and ABC News describe how a history of disenfranchisement has led to mistrust of mail-in voting among many Black Americans.