Well, even getting USPS going on Zen Cart so I can see what I need to do is proving to be a small challenge.
USPS does require registration and they quite helpfully provide you with a test server and then switch you to their production server after email request.
Way more helpful than UPS!!! You can guess what I think the S in UPS stands for
At this point I suspect the issue is having a real address to provide, so I'll try that tack and see how it goes.