I too noticed the new checkout options this morning when looking at the Australian book offer. There was an option to use regular checkout which I did but as I clicked through to that I noticed the quick buy check box was ticked by default to quick buy from now on and I think had I gone to purchase another book I too would've ended up not being able to enter a promo code
I have since gone to my account settings and deselected quick buy
Why anyone would want to use it if you don't get to enter the promo code is beyond me