I have a Wordpress plugin that I developed a couple of years ago that uses the free MaxMind country location database. It resolves the visitors country from their IP address and redirects them appropriately.
I have re-written the plugin to be stand-alone so it will work with AOM and it seems to be working so far.
Here is the structure of my test site: (AOM 5.4 Full Version)
http://www.dwallace.net has an index.php file that will redirect you to one of three AOM stores (US, UK, or CA).
/shop/index.php (Amazon US AOM Store)
/shopuk/index.php (Amazon UK AOM Store)
/shopca/index.php (Amazon CA AOM Store)
The script in my main index calls the free MaxMind country database to determine your country, then redirects you to one of the three sub-directories above. If you are not in the US, UK or CA, you will get a blank page, but it can be easily changed to direct people anywhere you like. This may be useful for cutting down on unwanted countries viewing your sites.
Once you are redirected to the AOM store for your country location, the MaxMind database is called once again. That way if you come to any of the three stores directly with your browser, you will still be redirected to the proper store for your country.
Try it. If you are in the US, try going to the AOM store for the UK . You will be automatically redirected to the US store. Likewise, if you are in the UK and you try to directly load the CA store, you will be redirected to the UK store.
http://www.dwallace.net/shop/index.php (US Store)
http://www.dwallace.net/shopca/index.php (CA Store)
http://www.dwallace.net/shopuk/index.php (UK Store)
Let me know if this is working for you and if you would like to have it for your AOM stores. I'll zip and upload the files and explain how to install them.
Daniel.