h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

My guess is that mail would have been delivered, at least partly, by stagecoaches, which were pulled by ponies. My grandfather in Michigan worked for the Post Office, but not early in his life. By the time he joined the Postal service, deliveries were being made in Fords, which he service provided. There was a running joke to the effect that he could have any color as long as it was black. Possibly, in 1910 horses were involved in mail delivery in Hooverville, but they weren't part of the pony express....

You're right, the Pony Express and Hooverville never met. However, there's an old highway marker right in front of the courthouse, and if you've ever read 'Coming of Age in Brookville' you know that the coaches came through and delivered the mail back in the 1840s.

We even know the postage prices and what happened if the drivers took the corners too sharply: A88004289 This would never happen with a postal vehicle nowadays.

There's a vintage dinnerware pattern called "Royal mail," that shows a coach and horses stopped while the mail gets delivered. There are different colors, but the blue has always appealed to me.

http://www.replacements.com/china-myott-staffordshire-royal-mail-blue/c/64694

Come to think of it, I have four salad plates in the pink version of the pattern.