Not sure how much I want to let folks know, but you can scrape the routes from https://eddm.usps.com/eddm/select-routes.htm (backend you can just check out the ESRI server it hits in DevTools, it is just an open endpoint).

So those are not "polygons", but the actual polyline routes. But if you get that far you can probably figure out the GIS operation you want to do for your use case.

Those are the more specific routes as well (so more specific than the census ZCTA), although ZCTA should be good enough for most folks anyway to be honest.