Most of the retail world leaves customs or import duties to the purchaser. They are responsible for any of these charges, so you can ignore them. However, you must say explicitly in your terms and conditions that any charges are down to the buyer. You must have the correct customs declaration on your goods but your shipper should be able to help. If your sales grow you will learn more about this whole area and be in a better position to advise your buyer. There are different procedures for trade sales, so you would need to investigate these if you are operating in this field.
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