The hard part about implementing a distance calculator based on zip codes, vab, is not the code; the actual code to do that is so easy, it's almost not worth the effort to find someone else's implementation for the borrowing. No, the hard part is getting an up-to-date zip code database that'll map to useful coordinates (such as latitude/longitude) that can be used in distance calculations.
I notice that Zipdy's description says it uses a database "created by the US Census". Does this mean updates are only available for this database once every ten years? The customers I've worked with who were using Zip codes for commercial projects (e.g., on their websites all seem to think that having all Zip codes in the database is important enough to pay a $1000 annual subscription fee to ensure it's updated quarterly. Certainly, though, Zipdy is a nice resource for those working within more limited budgets.