Ok, I have a fix for this:
The problem is caused by a program called "AppSync" common on jailbroken devices. Unfortunately, this workaround means you will lose all your characters and saves, but I'm sure there's a way to workaround that issue, too.
If you have a jailbroken iPhone with AppSync installed (under the Cydia or Rock interface), here's how to purchase additional levels and maps:
● Uninstall AppSync through the Cydia or Rock interface
● Uninstall Oubliette (NOTE: This will delete all your characters and saves!)
● Reinstall Oubliette
● Reinstall AppSync
● Spend away!
John/Jim: I suspect there's some way to fix this at the app level in a future update, as it's some variable/setting being picked up at install-time. Some other apps with in-game purchases like We Rule do not exhibit this problem, while other apps like Navigon *do* have the same issue. Sorry I don't know much more about it!
EDIT: Logging out of iTunes account did not fix the problem for me, unfortunately - it may for some, so try this first!