When zippopotam.us 404s on a real US ZIP: building a 4-tier geocoding fallback

ZIP-to-lat/lng feels like a solved problem. There are something like 41,000 active US ZIP codes. They're a public dataset. They're geocoded. Multiple free APIs serve them. You wire one up, it works, you ship. That's what I thought. Then a user reported that the gas station on his block was showing as "8,247 miles away." Quick context: I run a gas price finder side project. When you enter a ZIP, the site looks up that ZIP's lat/lng, then sorts nearby gas stations by driving distance. If the ZIP...