The story stays Zavvi says "We have tried to contact you on numerous occasions to give you the opportunity to return this item to us (at our cost and no inconvenience to yourself)."
So let's use some common sense here. Assuming they were telling the truth... if a company called me and explained the error, apologized, issued no threats, but ASKED me to send it back, offered to send me a prepaid return sticker, and offered to schedule a pickup if I didn't want to take it to a dropbox, I wouldn't fuss. To me, it would all be about the amount of work I'd have to do to return it. Make it easy for me to do, sure I'd do it.
The legal threats are stupid. Their percentage of returns acting nice will be as large or larger as the results of acting nasty. And if they take legal action they'll not only occur expenses, they'll turn every one of those customers into enthusiastic broadcasters of ill-will.