If you asked them.
And they didn't make a big fuss about pointing this out to you - the details were buried quite deep in the help pages.
But it is an excellent idea and it was a reason for me to use them.
As I've just seen something I bought a few days ago drop by £9.90, I looked for the details on the site. Couldn't find them. I remembered that they weren't exactly prominent, so I sent off an email.
Apparently, this promise was dropped on the 1st September this year.
Now, they're going to give me the refund (thank you Amazon) but can anyone find anything about this promise going? Their terms and conditions reckon they were last changed in 2007, for example.
I can't find anything about this on the main Amazon.com site either...