Quite a lot of unhappy people on FB below I attach the gist of my comments which basically say it was not illegal just very sharp practice:
... depends when the contract is made. If they offer (invitation to treat) you then respond with your order the contract is made when they accept your order but not before. In that case they have not done anything illegal although probably very poor marketing and customer retention. The same is true on the high street, by the way, the price is an offer which you accept (or reject) and the contract is made when they accept your offer to pay the price stated. they can always refuse and no contract thus nothing illegal.
... it might seem false advertising but they will be able to claim "unavoidable mistake" and wriggle out of it. Poor marketing, goodwill and so on but a trick of the trade I am afraid with no legal redress. Loads of case law will be on their side even though it flies in the face of what most people see as "justice" which is not quite the same as the law of contract. Unfortunately often offers that are too good to be true are in deed that - there is an old legal epithet "caveat emptor" which means "let the buyer beware" and that is the rule of thumb to follow. Amazon care about reputation but not everyone else also Amazon would tend not to make such errors given it is a very slick and efficient organisation. I make a note of any doubtful traders and always ignore totally. I can recall having this sort of discussion with students when talking about law of contract in a business context.
As stated above it is a common misconception and one we had to correct with our business studies students when they did their mercantile law (contract and tort - all the boring non-criminal stuff) as the point about when a contact is made is crucial. Once contract made getting out of it difficult but so is adding new conditions and terms!
But there is a lot of this false advertsiing that is blatant - a bit like some political statements really!
