The driver comes to the door, your dog barks, and the driver leaves. From your dog's view, that sequence can seem to work. They may be worried, excited, or reacting to an abrupt noise. You do not have to decide which one while holding a package in one hand.
For real deliveries, put your dog behind a gate or in a comfortable room before opening the door. Keep their access to the door and the delivery person safely separate. Add a delivery instruction to leave packages without knocking if the service allows it and that fits your home. A courier should not be asked to feed, approach, or train your dog.
Practice on a quiet day
Teach "go to your mat" away from the front door. Reward the trip, release, and repeat in easy sessions. Next, have a household member approach the door without ringing. Reward your dog for going to the mat while the sound is mild. Over time, add a knock or a quiet recording, then a familiar helper at the door. Keep the dog at a comfortable distance and the helper outside unless you have a safe visitor plan.
If your dog cannot eat a treat or respond to a cue, make the exercise easier. Don't keep ringing until they stop barking. Real deliveries will happen while you're still training, use the gate and let the practice resume later. If your dog has charged, snapped, or tried to get through the door, seek help from a qualified, reward-based behavior professional and tell visitors not to enter unannounced.
The first useful change is simple: the package arrives, everyone stays safe, and your dog has somewhere else to be. For a broader doorway routine beyond deliveries, the doorbell and doorway guide can help you plan practice between visits
Sources: American Kennel Club: Why Does My Dog Bark at the Mail Carrier?. Dogs Trust: Dog aggression towards visitors.