From the Washington Examiner:

According to a July 2020 analysis by R Street associate fellow Nick Zaiac, “although vote by mail requires a letter to be sent and received — two distinct movements per mail piece, both of which happen in a relatively short period of time — in an electorate of between 150 and 250 million, that is no more than 500 million letters.” Even if double that amount of mail were required to facilitate the election, the total number of mail pieces sent and received would be a drop in the bucket compared to overall USPS volume. In 2019, first-class mail (i.e. letter) volume totaled 54.9 billion mail pieces. Even an entirely vote-by-mail election would bump mail volume by less than 2%. And given that the agency’s mail volume has declined by more than 2 billion mail pieces per year since 2010, we know that the USPS can handle the extra load.