Hi tevahart,
It sounds like the wrong zip code is AutoFilling in Safari. We're glad you've joined us in Apple Support Communities. Let's see if this helps.
Are your AutoFill preferences correct in Safari and set to use your contacts? Go to Safari > Preferences > AutoFill then check and see if "Using information from my contacts." If not, let's check that. You can see more information about AutoFill in Change AutoFill preferences in Safari on Mac.
If the issue remains, let us know if this only happens with one website or multiple websites. Also, let us know if this happens when you start typing or if it's automatically detected. If it's when you begin to type, then the issue is likely with AutoFill. If the website is saying something like "Your zip code is #####," then the location shown might reflect the network or VPN you're connected to, and not your physical location.
We hope that helps with your question and take care!