From your dog's point of view, the letterbox routine is a perfect success story she performs daily: something rattles and pushes into her territory, she charges, barks, and bites at it, and the intruder withdraws every single time. She has repelled the invasion for months. Changing her mind means changing both the mechanics, so the rehearsal stops paying, and the prediction, so the sound starts to mean something better.
Stop the Daily Rehearsal First
Management comes before training, because every real delivery she 'wins' undoes your practice. Fit a basket or cage behind the letterbox so letters land contained and cannot be grabbed, shredded, or fought over, and so her teeth and nails can never meet the flap or a postal worker's fingers. An external post box removes the event from the house entirely and is the cleanest solution of all. Until training is established, keep her out of the hallway at the usual delivery time, behind a gate or door with something to do, so the real post arrives unopposed and unrewarded.
Stage the Sound Until It Predicts Treats
Practise at times she does not expect, using your own envelopes. Begin from outside: rattle the letterbox gently, slide one letter through, and at that exact moment, wherever she is, scatter a handful of genuinely good treats on the floor or call her to her mat and pay generously. Keep the first sessions quiet and easy, with her at a distance from the door, and repeat in short sets across several days.
Build the pieces in order: the rattle alone, paper sounds alone, then both together, then a person visible outside the door, then knocking added later if callers are also a trigger. If she charges the door during practice, the step was too big; make the next repetition quieter, earlier in the day, or further from the hall. What you want to see developing is a dog who hears the flap and turns to you or trots to her mat, because that is where the sound has started paying.
Graduate to the Real Delivery
Once staged post bores her, let her be present for the real thing, still managed: behind a gate within sight of the hallway, mat down, treats ready at the rattle. Pay the look-back and the settle, and accept that the first real deliveries may include barking; stay boring, reward the recovery, and shorten nothing in a panic. Over one to three weeks of most days going well, open the gate, then remove the gate, keeping the basket in place permanently, because management that stays is not failure, it is good engineering.
Letterbox training often exposes a wider door problem, barking at callers, guarding the hallway, or charging visitors, so practise door manners as their own exercise rather than assuming the post work covers them. If she guards the door with stiffening or snapping, or the behaviour is escalating, bring in a qualified behaviourist, since doorways concentrate arousal in a small space.
Make It a Household Routine
The plan fails if one person practises and another lets her ambush the hallway for the fun of it, so agree the rules and share the sessions out. KinsLe's Care Teams lets you assign staged letterbox practice and delivery-window duties to family members, with each completed session timestamped, so the training happens daily instead of whenever someone remembers.