FWIW, I actually have a custom module that administers a list of postcodes and makes conditions available - this is useful with the Flatrate shipping system (so you can "allow shipping rate X" if "delivery postcode is within Zone N"). It's kind of specific to UK postcode formats, so may require reworking before it'll allow zip codes, but I might be able to make it available for reference or mutilation if people are interested.

It's built for Ubercart 5.x-1.x, so somebody would at least need to change all the "workflow-NG" references to "Conditional Actions" (or whatever), on top of whatever other 6.x-2.x changes would be needed. Oh and there's little to no documentation...