As I'd been left with a dangling order or three, I emailed the support address. They got back to me to let me know the cause of the problem (I was using Flickr metadata on my cards, and due to the API changes this wasn't being parsed correctly) - and gave me a code I could use for a free set of cards for my trouble. Not only that, I was given a real email address I could communicate with, so I could give them more information if the problem returned.
I had another go at making some cards today - without metadata - and the order went through easily.
Now to wait for the cards to arrive!