We're working on this for a number of Zen Cart builds right now. We've done 2 Virtuemart & a CREloaded versions so far and have some Zen versions completed that are just not public yet.

You can see our methodology we're following for Washington State Destination Sales Tax here. If you can think of an easier way to get the zip + 4 from a user who doesn't know it ahead of time (like every customer in existence) we're all ears.

USPS wanted $3,000 for access to their real time API when we called ... so we went with Yahoo Maps to look-up the +4.

I guess merchants who got on a pilot program got a B & O tax credit towards their implementation of this if they were done prior to June 30th or so.

We'll post the Zen Examples as soon as they are up. The two we just completed have not launched their sites yet.. otherwise I'd show you how it looks.

Tom