ChatBefore they buy
Prices include / do not include sales tax, and the tax is shown at checkout before you pay. For orders outside your country, import duties and taxes are included at checkout / paid to the carrier on delivery.
It depends on the market. US prices are usually shown before sales tax, which is added at checkout, while UK, EU and Australian prices usually include VAT or GST.
The shopper, either at checkout when the store collects duties upfront, or on delivery when the carrier asks for them. Say which before they buy.
The carrier usually sends the package back. Decide in advance whether you refund it minus shipping, and put that in your policy.
Aide fills in the Shopify order and tracking, then drafts or sends the reply, as you choose.