The ballots don’t have to be treated like precious cargo between the time they are received back and scanned and when they are counted.
The barcodes ensure only ballots that were sent out to people weeks earlier, and returned by the same people, can be counted after the election.
Even valid blank ballots that were sent out and not sent back can’t be counted on Election Day no matter what you do to them.
If they didn’t get returned to the county clerk on time, where they were scanned as they arrived, they can’t be counted. That scan when they get back turns them into valid ballots. Before then they are just pieces of paper.