Did the P85D actually come to a full stop via regen? I thought that was only a feature of newer Tesla models. The newer models are certainly capable of stopping using the motor alone, you can feel the friction brakes engage to hold you at a stop.
Newer models have much stronger regen all the way down to 0mph.
The main issue with relying on regen is reduced regen power when the battery is cold. For a mail truck I think a stronger battery heater while plugged in, or a braking resistor to turn regen power into battery heat, would be helpful.