In addition to the fact that like smeyer mentioned that many USPS vehicles just stop and deliver through an area, or only go to community boxes now, using USPS

In addition to the fact that like smeyer mentioned that many USPS vehicles just stop and deliver through an area, or only go to community boxes now, using USPS vechicles would likely come with significant legal issues. The USPS is (in part) funded by the US Govt, it is essentially a government owned and operated division. So if you start jamming 65MP camera's with highly accurate GPS and inertia sensors etc on them may cause peoples heads to explode with conspiracy theories. I would see a more likely partnership being the trucking/shipping industry (like FedEx/UPS etc). While the technology would have to change some to get the right angles etc, it would seem like an easy way to cover significant territory. But then again, it wouldn't be contiguous areas at the same time so they would likely have to mash old and new images at a much more frequent pace which may not provide as good a consumer experience.