Charged for Audible account despite having cancelled it

I was offered an opportunity to download a single book after subscribing to Audible, and it was stated that I could cancel at any time. I unsuccessfully tried installing it on my computer, my phone and my iPad. All gave various errors associated with outdated firmware or the like. After trying for over an hour (and I don't give up easily) - I gave up and cancelled my account without having successfully listened to the audio-book. No receipt for cancelling the account was given to me, but I did receive a nasty surprise on my bank account at the end of the month. Of course, the invoice was sent from ********** I was cautious, but it sure looked ligitimate enough and endorsed by Amazon, so I'm surprised at their atrocious customer care.