There’s no doubt the US Postal Service is struggling. It faces a multi-billion dollar deficit, and is considering closing thousands of post offices. For years, the USPS has been complaining that email is eating into its market share. And they’re probably right. What you’re more likely seeing in your mailbox is exactly what I’m seeing: mountains of ads, address labels and catalogs you never asked for, and don’t want. All of this commercial detritus begs the question: How much is junk mail propping up the US Postal Service?...