I'd be ok with the changing prices--I use Amazon's subscribe and save, and the price fluctuations are reasonable. If memory serves, if the price is totally out of whack or the item is out of stock, I get a notification about it.

I've also never had to return a subscribe and save item, but I imagine if the policy was the same with these "pre-orders," there wouldn't be an issue, since subscribe and save has been in place for decades.