Case C-422/17: Judgment of the Court (Fourth Chamber) of 19 December 2018 (request for a preliminary ruling from the Naczelny Sąd Administracyjny — Poland) — Szef Krajowej Administracji Skarbowej v Skarpa Travel sp. z o.o. (Reference for a preliminary ruling — Harmonisation of tax legislation — Common system of value added tax (VAT) — Directive 2006/112/EC — Chargeable event — Special scheme for travel agents — Articles 65 and 308 — Margin obtained by a travel agent — Determination of the margin — Payments on account made before the supply of travel services by the travel agent — Actual cost borne by the travel agent)