Public mailboxes had fax machines built in so that people could send faxes while on the move. One such mailbox was present in Courthouse Square, Hill Valley and advertised a “.05 Second Service”.
Behind the scenes
Back to the Predictions spells the name of this fax service as Pac-Fax (with a hyphen, as here).
As the movie makers had obviously not foreseen the rise of the internet, and with the rise of email, it appears unlikely the U.S. Mail will set up such a fax service as that seen in Back to the Future Part II.