With real-time tracking of bus and delivery vehicle locations, I wonder if it would be feasible to work out a software solution that allows delivery vehicles official, coordinated access to the unused time slices. Much like an automated version of what air traffic controllers do for runways and taxiways.
So for example, when a UPS truck enters a neighborhood where it needs needs to deliver 3 packages, the UPS routing software could query the transit agency software telling it the 3 locations, and it could respond with time ranges that they're available. Bus #123 will be at stop #456 at 2:33pm, and that stop won't be needed by another bus until 2:44pm, so we can reserve the time slot 2:35pm-2:42pm for you to make your delivery.