Perhaps the problem is more that you didn't present them with an option to choose the auto-generated option.  I think a graceful method would be to allow them to enter a zip code, but have a radio that has the auto-generated option as a choice.  So they are presented with "Enter your location" which they proceed to click the radio box. Presumably you could then save the location they manually entered in via a cookie and present that as an option as well.