Their English-language website can be found on int.ert.gr, but their postal address doesn't appear. I can't remember if I posted this image in an earlier thread, but here's a Voice of Greece QSL card I received in 1980 by snail mail. It includes their postal address, although that Wikipedia page shows that the address has changed after all these years.
Good luck getting any mail from abroad during this pandemic. Even domestic mail in Canada is backlogged.