The EU has a trade agreement with South Korea, enabling preferential trade. However, for shipments over 6000 Euros, normally a EUR1 would be issued to certify the goods meet the rules of origin. However South Korea does not accept the EUR1 form.
For preferential trade with South Korea, then Approved Exporter Status is required to enable preferential import duty. To apply for this, you need to complete a C1454 form and once approved you would use your Approved Exporter Number for your customer to benefit from preferential.