Those earbuds were specially delivered after I queried the Fliway helpdesk. Their website hadn't told me the parcel had been passed on to Courierpost, with its own tracking number. Apparently Courierpost claimed it had tried to deliver them several times, but I don't believe that at all.

@Miraz Yes, those “Recipient could not be reached” messages are a bummer all the while you sit home and wait for the people to ring the doorbell or your phone. 😒