Do the provisions of Article 28 et seq. of Council Regulation (EEC) No 2913/92 permit an agreed transfer price, which is composed of an amount initially invoiced and declared and a flat-rate adjustment made after the end of the accounting period, to form the basis for the customs value, using an allocation key, regardless of whether a subsequent debit charge or credit is made to the declarant at the end of the accounting period? If so: May the customs value be reviewed and/or determined using simplified approaches where the effects of subsequent transfer pricing adjustments (both upward and downward) can be recognised?