Ok, if it’s FBA export, then according to their own blurb, there should be no extra charges.
Take a look at:
Specifically, the paragraph:
FBA Shipping Programmes and Export (‘FBA Export’) allows you to fulfil orders of eligible products on Amazon.co.uk to a postal address that is not within the UK. With this programme, you can expand your business to more than 80 countries without dealing with the complexities of selling internationally, at no additional cost or effort for you.
You need to chase them for the money back. But you also need to clarify just how exactly it has been sold.