Customs exemption limits for postal shipments vary greatly. For European Union it's 22 EUR, for the US it's 100 USD, to the best of our knowledge (disclaimer). Other countries may have other exemption limits.
Cargoes over the limit values will usually go through customs control. You may be asked to provide documents (invoice, payment proof etc.) and pay associated import fees (including taxes and customs charges). Import fees are usually based on the item's price, shipping weight, dimensions and country of origin, and may also involve customs brokerage fees.
Customs authorities may stop items that are under limit values too. But when you submit your documents, including payment proof, items are usually released without payment of any tax.
Customs procedures and costs differ from one country to other. Since import fees depend on the customs practices of the receiving country, there's no fixed import fee that we can incorporate into our prices.
Therefore, our prices do not cover any import fees, and buyers are responsible for responding to customs and paying import fees, if any, when they receive their item(s).