It doesn’t say how big and expensive it has to be does it?
An email saying “Your letter is at the post office in town, come and get it” would be perfectly acceptable.
In the 1800s the local news paper would print the names of those who had mail at the post office. There’s one in the Shelbyville, Tn paper that names David Crockett. At the time he was on his way to Texas. Ne never picked up the letter though his family may have.