Sealed vs Unsealed has more nuanced definitions than just "letter"/speech and "item"/things, though. If you look a bit further past the quotes you provided, you'll see:
>Sealed mail includes: First-Class Mail; Priority Mail; Priority Mail Express;
Shipped packages ("things") can be (but aren't always) sent as First Class or Priority Mail, and thus would fall under the "sealed" category as well.
OTOH, you can also send normal letters (not periodicals or marketing material) as "standard mail" which would make them unsealed (I don't know why you would do this as opposed to sending it as First Class or Priority, but you can).
Basically, USPS provides a service where, regardless of what you are sending in most cases, you can choose whether or not you want it protected by the 4th amendment. The origins of this "choice" (and why it isn't the standard to always being protected) is where I am more confused. And when talking about parallels to electronic media, I think this is an important point, because if this is the case for physical mail, it also stands to reason that electronic media for some reason might not default to being protected by the 4th amendment, either.