My experience with the postal tubs is that they are probably missing because the mail carriers don't care to bring them back. At least some of them. At my last job they would drop off tubs of mail. They would be put by the door and the mail carrier would not take them back. We would do mass mailings and fill them up and take them to the post office and that was the only way they got returned. I was seriously tempted to take a couple after they sat by the door for days, but never did.

What I would really like are some of the old milk crates that were made of metal wire.