Sigh.

Well, I found a solution on my own. There's Drupal 6 contributed module called Validation API that lets you add validation to any field. The good news is, it solved my problem, at least short term.

The bad news is that it's incredibly cumbersome to administer, and I can't really see talking my client through it. So it's a terrible long term solution.

I'm brand new to Drupal/Ubercart, and as I said above pretty surprised this basic feature isn't easy to implement. Hopefully I've helped somebody else with this thread, since everybody using Drupal these days isn't keen on developing modules (as enjoyable as that may be for some of you).