E-Commerce Invoicing, Billing and then...Cancellation?

This understandably irate Anonymous Coward asks: "I recently ordered some memory from an on-line retailer for an insanely good price. It turns out that the price was *too* good -- claiming ' a mistake' they backed out of the order (I had assumed 'crazy promotion' and ended up buying other stuff as part of the order). However, not only did their cancellation come after I'd received a full invoice AND receipt, but my credit card had already been billed. Adding insult-to-injury, since I live in Canada, once they had refunded my CC amount, I was out a few bucks on exchange rates and fees! This isn't the first case of this I've seen - buy.com seems to get into price 'mistake' fiascos all the time, and Argos in the UK had a TV-for-3-pounds gong show last year. But where does the responsibility lie? In my case, I'd been fully invoiced, charged, and receipted before the cancellation. IANAL, but isn't there some point at which the invoice becomes legally binding? I do know the old bit about 'if something sounds too good to be true, it probably is' but I also know that if I sign an invoice or click that 'I Agree' button, I'm in a contract. Shouldn't the other party be, too?"