No doubt, the dog sees the postman leaving again, as a victory. In the dog's mind, the dog has just successfully defended his home, and caused the invader to go away.
Our mailbox is at end of the driveway, too far from the house to interest Buddy. But, no doubt, if our mailbox WAS attached to our home,
Buddy WOULD bark at the postman, or anyone, stepping onto our porch. I don't see it as a problem, really,
i WANT my dog to announce all visitors and all trespassers.
I feel this is one of the main
benefits to having a dog in my home, is the added security to our home, as well as being an "extra doorbell".
Actually, when i first got my dog,
he did NOT bark for visitors arriving.

and i specifically trained him to do so, to bark when anyone arrives on the porch, knocks on the door, or rings the doorbell. I trained my to bark for those cues.
It's his "job".
But, if you DO want your dog to NOT bark at the postman, that could be done. You'd have to consistently sit beside Merlin, for each time someone steps on the porch,
to give cue to "shhhh" and reward Merlin's brief moment of silence, and never allow Merlin to bark at your postman.
You could use volunteer pals, to arrive at your door, and use those moments to teach Merlin you no longer want him to announce visitors anymore, by giving cue to "shhhhh" or "silence", and rewarding the moment of silence.
He'll bark again, repeat cue, and reward the moment of silence. Praise dog, massage his back, speed-feed treats while he is not barking. Really reward that moment of silence, as this is reeeeeeally difficult for dogs to do.
Overtime, slowly extending the moments of silence from Merlin til he gets treats, slowly desensitizing Merlin to the sights or sounds of someone on his porch, teaching him you want silence for those moments.
Might be better ways, but, that is how i got my dog to stop barking at the neighbor's dog.
Every time Merlin IS allowed to bark away at ppl arriving on your porch---------- it is so self-rewarding

, it UN-does all your efforts to get Merlin to stop barking for visitors arriving.