What percentage of the city would have easy access to tunnel endpoints? It's a pretty difficult last-mile problem.

At least in the short term, a good bet would be funneling stock to retail stores. In lots of neighborhoods, big trucks parked on narrow streets to offload cargo is a big problem.

To do this you would need buy in from all the legacy carriers first. It doesn't work without having them all as your partners, so you get the economy of scale you need. That will be very difficult. Next, you need to consider your own operating costs. How are these vehicles going to be powered? Routed? You will still need to sort and distribute the packages. What if one breaks down in route? How are you going to lift them back up onto the surface? Are you planning to retrofit buildings with tubes going down into your network? Or distribute in/out terminals throughout the city? Both which would require an insane amount of upfront politicking and planning. There will be issues of easements, zoning, and liability to work out. Then, even if you get all the players to agree to your plan, which is unlikely, you still need to deliver the package at a (significantly) lower cost structure than they currently are able to do with government subsidized roads.