Zones for shipping costs and time estimates are pretty large, so they could even give rough estimates until you input the exact address. Sales tax can differ from street to street, but even that could probably be guessed to within 1% based on your browser's location.
When I lived in eastern Idaho, geolocation APIs regularly placed my location in Boise, over 300 miles away. They work fine for people in larger cities, but aren't precise outside of that.
Yeah, I live in NJ, but I work at a federal facility with an IP that geolocation places in DC. So I frequently see sites assuming I'm in DC when I access them from work rather than home, even though the two locations are actually three miles apart.
So I certainly prefer when they just ask for a ZIP code.
