You are QUITE RIGHT, Kevin. Don't be QUIET any more. Though not as actually serious as this, I have had quite a few instances of badly delivered mail and I had had to complain - and bitterly, and with stern advice!!! - on four occasions in the past 2 years.
You just DON'T get what they purport to offer as a "service" and what you pay for! At least, not always, so that when you do it seems almost like an accident.
This is the official complaint form that's available online.
Oh, how humble and contrite are the responses I get back from them. As if that will help. However, I think they have pointed out to the local postie the folly of his ways, because it hasn't happened for a 2ple of months.
Good luck, and if you need any epithets to hurl I would be glad to oblige.