I'd probably try and go from the angle that you bought it from the store, went home, checked their site to find that you had been overcharged. Perhaps ask them why the price on the website is so much cheaper than the store as it's the same company?
Failing that you could scratch the cd's, take them back and state that they were damaged and ask for a gift voucher (assuming they don't have anymore in stock)
Anyway, aren't all consumers entitled to time period where they can return goods under the Consumer Goods Act or something?
Perhaps check their website for details of their returns policy?