The point is that mail-in ballots are time-sensitive and the USPS has already fallen behind on its current workload.

I've seen tweets of e-tailers saying some packages (very small+light first class... the kind that are only cost effective thru USPS) are taking weeks when they used to take 1-4 days.

Unless USPS magically becomes more efficient in the next 100 days, their backlog of work will grow and non-prioritized deliveries will fall further behind. It will especially hurt voting is Amazon (or other packages) have higher delivery priority than ballots and ballot materials (usually 4+ items per voter per election).