When goods are not shipped domestically (within your country) or within a single customs union, such as the European Union, you are responsible for any inbound duties and taxes which your local customs authority deems appropriate. 

These fees are in addition to the price and shipping cost you are charged when checking out.  Applicable duties and taxes are calculated at the border and based on the nature of the item, final price of the item, and the destination country.

For U.S. import questions please refer to the Customs & Border Protection website.

For EU import questions please refer to the European Commission website. 

For UK import questions please refer to the UK Government website.

This is an example of a response that you might give. It's good to be as thorough as possible in responses as that has a tendency to improve trust overall.