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).

Delivery is based on routes, which are designed to take 8 hours to fulfill on average. If there's a heavy parcel day, bad weather, coverage for sick workers, etc, the 8 hours may be 11 hours.

Now, they stop working after 8 hours in most cases and leave whatever is left in a pile to be done later. Just like with computers, when you develop a backlog in a system optimized for throughput, things tend to get worse.

Critically, mail in ballots are generally not accepted late. USPS will get everything where it's going eventually - but if the Trump appointee announces an impromtu postal holiday the week before the election then

1. Probably nothing technically illegal has happened

2. Nobody's mail-in-vote will arrive in time to be counted

This coupled with the fact that those rabid Trump fans will be showing up at the polls in person might[1] be enough to tip it in his favor.

1. It still might not be enough though, he is doing hilariously abysmally in polls.

As Wisconsin demonstrated, the attempts at voter suppression will likely backfire. If the Republicans had agreed to mail-in balloting or a delayed primary, they might have held on to the state supreme court seat they lost, but what they ended up doing kept Republican voters home (why go vote in person in a pandemic when there's no top of the ballot choice to make) while the Democrats stood in line (between the obvious attempt to make them stay home [Streisand effect] and the real impact of their top of the ballot choice). I'm just some schmuck in another state and I could see the likely outcome of the attempt at voting suppression before it happened, but Republicans seem to have only two tools in their toolbox anymore: tax cuts and voter suppression, and those tools aren't doing the job now and hurt more than help.

Doesn't your mail in ballot just need to be postmarked by election day to count? Its not the day its received, its the day its sent that counts.

probably depends on the state?