I'm still holding out hope for ballistic delivery from UP/Ex launchers, as envisioned by Vernor Vinge in Rainbow's End. A solid fling and a guided parachute might actually work. Major airspace sharing concerns, of course.
No need to go that high at least initially or even use parachutes. Just mount your system that lobs packages on a truck. It just drives a route and some software lobs everything where/when needed. Use a special sticker that's reflective in a particular spectrum for the truck to target and give the stickers out for free. Have the truck scan for the stickers, record gps coordinates (so you know which addresses are deliverable with your new system) and scan for obstacles so you can compute potential firing angle (too low and you're in the shrubs, too high and you're in the power lines or tree branches) as it drives around. Once you know the potential firing arcs for a supported address you can calculate what packages can be delivered without going too high/low or over/under shooting. Even if the truck has to stop to shoot it's still a labor saving improvement over traditional delivery and the sound of a low pressure pneumatic cannon isn't any louder than a truck door. You could use existing warehouse infrastructure to make sure that packages that are not of the right density (or are market in your system as too fragile)to safely toss don't wind up getting tossed.
You're basically just using software and data input to replace the judgement of the paper boy (hard) and some powered rollers and actuators to replace his arm hard (easy).
Disclaimer: I spend 3min thinking about this, this is by no means a serious pitch.
I can definitely agree with the theory behind it. Pair autonomous systems with delivery trucks and let them deliver easy packages (light weight, non-fragile) and let them deliver from the truck to harder addresses (long driveway, up a flight up steps in a breezeway) or deliver ahead.