Well... I just saw that my bank account has been debited of 79£ (99.37€) by Amazon.co.uk for a year of "Prime membership" that I can't even use because I actually live in France and use Amazon.FR where I had deleted that so called membership before the "free" month stopped. I thought it would be ok since it's the same website and all... Biggest mistake ever.
Edit: Problem solved! The Customer service answered quickly (just a couple of hours) and now everything's back to normal. At least I don't have to stop buying books ! ;)