Turns out the error was mine, but Public Mobile couldn't recognize the problem over the course of 20+ messages either.
I accidentally clicked and bought '1 Gig of Canada Data' rather than '1 gig of US data'.
My bad, so I can't blame them too hard for not being able to find out why the US data wouldn't work. In the end, I wish they would have been able to deduce why it wouldn't work, but it was me that clued in, once I saw the option to purchase the package I thought I had was still available - knowing they disappear from the list once you've purchased them.
All's well that ends well, I guess.....
[To err is human 😊 ......... Luddite]