SirPrimalform: The 5 minute window is a good idea, but there needs to be some warning if the average has risen.
It's very much not a good idea imho, I doubt that it's even legal (with or without notification, though that probably depends on the country they're in).
If they want a 5 minute window, they need to disable the buy button after 5 minutes.
They can't bait their customers with one offer, and then switch part of it while he's in the payment process.That's immoral and legally questionable, even if they are just being naive (which is the most likely scenario imho).
However, I have to say that with their contradictory definition of DRM-free, and now these bait-and-switch tactics, my appreciation of their service begins to dissolve. Plus, they didn't have so many good offers so far anyway.