Please remember the www is international, "the post office" is highly ambiguous.

Also, just because something could be private doesn't mean it should be run for privatised gain. I'm pretty sure the UK post office was recently privatised.

And indeed a privatized USPS could potentially be considered a common carrier and regulated as such, although note that currently neither UPS or FedEx is.

But that has no bearing on financial services (like square or visa, which are all contract carriers) or hosting companies (also contract carriers), or other things like that. ISPs are the one exception which might fit the bill of a common carrier.

The important difference is that an attempt to speak at unwilling subjects via the mail would constitute harassment with common carriers. They're not broadcast mediums since they are point to point.

Yep, I'm not actually sure if there are any examples of common carriers that aren't point to point. Radio isn't a common carrier, you have to be federally licensed and its highly regulated (and for example all of these youtube videos that we're talking about would result in the makers being banned and fined).