Internet shopping gets a little complicated in law - an automated receipt does not count as a "proper" receipt by law, so they can withdraw the offer after that has been sent out. A price on a website (or a price in a shop for that matter) is only an invitation to treat - they can still refuse to sell it to you at that price.
Can you tell I've got caught out on this before?! (When a website mistakenly keyed in a laptop price at £1!) We have got some bargains in the past though where the company made a genuine mistake but decided to go ahead at the cheaper price as a gesture of good will, though.