The Letters from No One

Did you notice?

The mail will not stop. A hidden world tries bureaucracy first: letters, then more letters, then a house that flees the post. The comedy is escalation. Under it, a family is working very hard not to let a child read his own name.

Foreshadowing

Owls as infrastructure show up for the rest of the series, including post that shouts and post that should not know where you are. The refusal to explain is also a Dursley method the books never really drop.

This echo points at book 5. It is a reading note, not a quote.