Customs fee - shipping from china to u.s. - who pays? what about customer refunds?

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  1. if a u.s. customer buys from my u.s. online store and the order is shipped from china to a u.s. address, might the u.s. customer need to pay a customs fee in addition to the shipping fee?

  2. if yes, is there a way to know what that customs fee would be?

  3. what if u.s. customer wants a refund?

trying to minimize negative surprises for me and my customers

thanks