Magic Mailbox Game

At the end of the street stands an ordinary-looking mailbox, but any letter or parcel addressed to anyone who ever lived will be delivered to their door the next day. You can only post one letter/parcel per day and it must be able to fit through the flap of the postbox

What would you post, and to whom?

I would post a 21st century structural-engineering textbook to the 1st century BC Roman architect, Vitruvius