I bet they don't fulfill the order. Usually an e. and o.e. exception is applied. I haven't looked to see if they have one. It might be difficult for them if they don't !
p.s. I just had a quick look. This is in their terms page, and seems to cover the situation.
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Pricing
We try to ensure that the prices indicated on the website are up to date and accurate. However, we accept no liability for any errors and omissions and the final price of goods is the price in force at the time of us accepting your order. If a product has been listed at a lower price by mistake and you have ordered that product, we will inform you as soon as possible and give you the option to either reconfirm your order at the correct price or to cancel it. If we are unable to contact you we will treat the order as cancelled. If you cancel and have already paid for the goods, a full refund will be issued.
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