I've packed it up and uploaded to my server - you can
download it and give it a try. Please make sure you create a backup of your site's files and database before you make any changes - I don't take any responsibility if anything goes wrong.
I currently don't have the time to write the documentation for it and submit it to the plugins section so I'll leave it like this for now and hopefully submit to Plugins when I get more time.
All changes are commented so you can easily merge it into your existing files. If you haven't made any changes to any of those files, you can simply overwrite. Run the SQL patch from admin and you'll be able to modify the surcharge from admin->Configuration->Shipping.
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