Here's Amazon's reply to my request for them to take Google XML feeds to their Product Ads service:
I am sorry to hear that you are experiencing such issues.
While we do try to make the process of using Google feeds as easy as possible, all our XML XSDs and feed processors are optimised to provide our databases and systems with necessary information, to change processing format would require a complete overhaul of all our systems which we are not currently in a position to make such a change that would allow Google XML to be processed without any conversion.
While we do understand your frustration with consistently modifying a feed so that it is compatible with any external system as it is not only time consuming but it can be quite frustrating. We would recommend to have your system produce the information in the Amazon format at the same time that the Google feed is being generated.
I do apologise for any inconvenience caused by this but I do hope that this helps.
They do not take XML feeds and only deal with CSV or TXT format. as you see, they believe that the onus is on the developers of shopping cart platforms to create the facility to export product feeds to suit Amazon in the same way as platforms already tailor their feeds for Google.