I bought an item online, now they refuse to ship it to me at that price and want to cancel or make me pay more?

So I bought an item off of AjMadison.com and I paid by card over the web and got a receipt email. Then they called the next day and said they made a mistake and they meant for the price to be over 100 dollars more and they would be happy to sell the item to me at that price or cancel the order. I feel like since I already paid and have a receipt for that item that despite their "error" that they are entitled to fulfill the contract and send me the item that I paid for. Legally, what are their obligations and does anyone have any advice for me in regards to what to say while contacting their customer service and not only complaining, but talking possible legal action?