From their Terms of Service...
PRICES AND AVAILABILITY OF PRODUCTS
Prices and availability of products on the Site are subject to change without notice. Errors will be corrected when discovered. Our Site contains a large number of products and it is always possible that, despite our best efforts, some of the products listed on our Site may be incorrectly priced. We will normally verify prices as part of our dispatch procedures so that, where a product's correct price is less than our stated price, we will charge the lower amount when dispatching the Product to you. If a product’s correct price is higher than the price stated on our Site, we will normally, at our discretion, either contact you for instructions before dispatching the product, or reject your order and notify you of such rejection. We are under no obligation to provide the product to you at the incorrect (lower) price, even after we have sent you an Order Confirmation or a Shipping Confirmation, if the pricing error is obvious and unmistakable and could have reasonably been recognized by you as a pricing error.
On occasion, you may be able to place a product in your shopping cart and submit your order for processing, but your order is subsequently cancelled due to unavailability of product. You acknowledge that products may sell quickly and there may be a short period of time after an order has been submitted, but where the product is no longer available. You agree that we may cancel your order after you have received an Order Confirmation without penalty.