If I remember my law class in the UK, there is no legal reason to ship the goods in this situation. The price online is only one that is offered. The buyer giving money to the retailer is considered an acceptance of the offer, but the contract is not binding unless the retailer then displays that they still wish to go ahead with the transaction on those terms. My lecturer always referred to it as the courts wanting to see "a meeting of minds."

This leaves online retailers (and brick and mortar that sticker an item wrong) the time to change anything before the item is placed in the buyer's hands, and the court will look particularly down on a transaction where the buyer could not have reasonably expected the transaction to be valid (like buying a TV for 1 GBP).

My understanding was that much of contract law in the US was based on UK law, but I am by no means a lawyer in either jurisdiction :)