If anything, postal remailing would probably only work in a TOR-like manner, with many, distributed, non-for-profit remailers - but that opens a whole set of new problems, like who pays for the service, what prevents the remailer to just take the delivery for themselves (as undoubtedly such a service would be used to remail illicit substances and other valuables), and how would such a network of legitimate, trustworthy remailers know each other to do some tunnelling?