It is your client who is responsible for paying custom duties and taxes. They may vary from country to country as taxation rules differ widely. There is a default page named Returns, which contains essential information regarding returns and refunds, available in the built-in themes. The information on this page can be edited through WordPress settings: Dashboard → Pages → Refunds & Returns Policy.
Custom duties and taxes

Written by Yaroslav
Updated over a week ago
Updated over a week ago