Chundles,
You are free and clear. I've had many times where companies have done similar things. A trash rolloff company, JC Penney, Overstock, the City of Minneapolis (this was the big one), etc. have all given me "free" service/products since they screwed up and told me to not worry about it, or ignored me for too long. (it's been 6 years for the trash company, I don't think they are returning my call) Go ahead and use those batteries if you have need. I believe there is even legal precedent for keeping extra product sent to you (not mistakenly delivered to you with another person's name on it), you are not responsible (meaning paying for shipping) for sending it back or buying it. It would certainly be ethical to send it back if they ask for it and pay for that, even after a year later.