Completed - not a perfect option, but it works within the parameters of machinable products i.e. I haven't done the fancy footwork on padding, tares and numbers of boxes.
Feedback on its accuracy would be appreciated.
Available Here
My testing on a couple of servers confirmed that installs, upgrades and functionality worked as required.
However, on a Site5 server, the use of either UPS or USPS resulted in a server 500 error and generation of a core dump file for every test, apparently from some mis-configuration of their provision for fsockopen().
It's a Site5 server issue and getting useful, or even timely, support from them is worse than pulling teeth, so I'm not chasing it. I don't use their service for anything important these days (still have 3 out of 6 months of free hosting remaining after their last disgraceful cock-up where the server was offline for 4 days - small exaggeration, only 92 hours, but who's counting, they obviously weren't counting and I obviously didn't count).