[inquisitor]

If you had € 20.01 left and you ordered the Internet Flatrate XL, your credit would have dropped to € 0.06 in the moment when the fee for the flatrate was deducted. In this moment you would also have been notified of your credit dropping below € 3.
I suspect you have started accessing the web before the Internet Flatrate XL has been activated (you get a SMS notification for that). By this you may have reduced your credit to less than € 19.95 and so the Internet Flatrate XL could not be activated any longer, thus data was billed volume-based in the standard tariff of € 0.24/MB in 400KB-increments (the big increment may be the reason why you were billed more than the actual data volume generated).

The USSD-commands should work on any roaming network. Has MDMA actually shown reception of a roaming network when you tried it? Have you closed Mobile Partner before starting MDMA? If so, there could be some issue on the current roaming network - then try another French network by doing a manual network selection through Mobile Partner (under Tools | Options | Network | Registration Mode | Manual Search and Register).