Since I didn't want to rely on a particular browser, I found and chose an existing standalone proxy application to investigate the queries. (Charles). All the mentions of standalone java-based proxy files seemed to have bad links yesterday. I appreciated the suggestion of a browser plugin, though!

One annoyance I found is that since I was developing OSX, the only browser which allowed its own proxy configuration was FireFox. (So maybe the Live HTTP Headers feature probably would have been useful after all!) Safari and Camino only go through the systemwide network proxy, so halfway through my debugging session, I was having to wade through extra proxy traffic generated by a few Dashboard widgets and such.

I got through my initial problem, though. Now to decide how to deal with the more complex portions of their shopowner site, where there's even more multi-page sequences and I have to carry item identifiers throughout the interface.