Post offices can close quickly. The USPS will never be good enough for mail in voting, I wasn’t meaning that.
Closing before the election simplifies the securing of votes. The National Guard or Marines can be called in to guard ballots and speed up delivery. Imagine a massive operation in which the entire postal service comes to a standstill for a day to ensure immediate secure delivery of ballots to their polling locations. Fewer post offices simplifies security protocols like cameras, etc.