Well further testing (on my part...support has been terrible at responding) shows the problem to be Firefox 3 specific. With IE 8, I can see "Custom Item Specifics" in a Fixed Price listing. I absolutely hate running IE but at least I've discovered a way to get some listings up.
Separately, Auctiva support has been very disappointing to work with both in terms of response time and competency. When this was a free service, I was accepting of those issues. Now that it's paid service, the this amateurish support is absolutely unacceptable.