Remember tax season in years past? The Post Office in the various towns and cities I lived in has a special "lane" set up for April 15th: you drive up, and you hand the envelope with your return to a worker. He then puts that envelope into a bin, clearly marked with the IRS office address.
If the Postal Service wants to go one better, they can do this: set up a similar collection "lane", accept the ballot, and insert into a sealed metal container clearly marked "Ballots". The entire container would then be delivered, unopened, to the counting area. The Post Office would not have a key, only the election officers.
This does not prevent some of the fraud methods that have been exposed in interviews with operatives. It does prevent the union postal worker from handling ballot envelopes multiple times on its way to the counting table.
I don't know if the Post Office has time to implement this idea; they may have to do with cardboard boxes with accountant's tape. But it can be done, and with little incremental cost.
Indeed, using this method would reduce the cost of collection and delivery, because several hundred mail pieces can be handled by the delivery service once, and without need for sorting (machine or hand). No need to postmark each mail piece, as they are bundled into a single container and delivered before the deadline, guaranteed.
Yeah, it will never happen.
This kind of happens in smaller areas. The ballots are pretty identifiable. They are culled out, stamped, and put right in the route for town hall.
But, now that local mail has to go to the big centers, this would be against the rules.
But we are too lazy too even leave our houses!