I recently was charged USD350 on an auto renewal for a service that I was no longer using.
They had sneakily defaulted the setting to auto renew and they did not send me an email notifying me that they'd renewed me for a year.
I only noticed it a few months later and I contacted them to ask for a refund -- they initially refused even though they knew that I had discontinued use of the service.
I found a tech site that reviewed the service that had a comments section associated with the article ... and I posted my review of their service noting specifically the auto renew setting and refusal to cancel and refund the annual fee. I took a screen shot of that review and informed them I'd find other reviews of their service and repeat the process.
They reconsidered and refunded me the fee.
One thing you will want to be sure of before you consider following this lead is to be 100% certain of your position --- if you are not then they could take legal action against you for defamation.
I'd first get something in writing from them detailing that they acknowledge they messed up and are offering $400 compensation... and that you have refused their offer and have demanded a refund or the proper item as a replacement at no charge.