I'm surprised they got away with rejecting the order. From my distant past working in retail the law says that there has to be an offer and an acceptance, and once that's been achieved, the contract is binding. Putting the Mac for sale on their site constitutes and offer, placing an order constitutes acceptance. Unless they've added something to the consumer protection laws recently it seems to me that the contract has been established.
A while back I ordered a very high end solder re-work station from a certain multinational electronics parts retailer (despite not really needing such an impressive piece of kit). They had it on their site for about 90% off normal retail price. I was expecting an email saying 'sorry, we screwed up' but much to my surprise a week later said piece of kit turned up. When I checked their site, they had fixed the price by moving the decimal point one place to the right.