If getting a Burofax is bad news, it turns out that there’s something even worse. You can go to a notary and for a large sum of money, he or she will go to the person’s house and try to serve the notice in person.
We only found out about the notarial notification a couple days later when a certified letter arrived in the mail, since the notary had obviously not been able to give us notice (since we were not there), so had just dropped it off at the post office. The notice gave us two days to respond.
So we trucked over to the notary and told her what we were responding about, and once we explain the situation, she was nice about it and was even willing to look up the delivery dates of the letters on the Correos web site and certify that the notices had arrived after the deadline.
The sender has to pay for the response, so on the positive side, we had a chance to have our side of the story certified.