True. Spot's going to get up-armored pretty quickly, even if it's just anti-tampering stuff instead of actually protecting against weapons.
Also, the delivery robots should be required to be safety green or orange, and have hazard stripes, more reflectors, and probably some kind of readily acessible, labelled emergency kill button.
i don't really know the most efficient protocol, but my gut says to treat it like a cop car or even a cop: open it up with whatever is handy and take a dump inside it.
Shotguns are pretty common, even in countries with strong gun control, and I'd wager a 12ga slug to the center of mass would have adverse effect on the bot