Okay, that is fortunately an easy one. You have joined INV1 and RIN1, which are the lines tables. However, you do not need them, so give it a try without:
SELECT T0.[DocNum]
, T0.[DocDate]
, T0.[CardCode]
, T0.[CardName]
, T0.[DocTotalFC]- T0.[DiscSumFC]- T0.[VatSumFC] as 'Total Sales ex GST'
, T0.[VatSumFC] as 'Total Tax'
, T0.[DocTotalFC]
, T0.[GrosProfFC]
, T0.[DocTotalFC]- T0.[DiscSumFC]- T0.[VatSumFC]-T0.[GrosProfFC] as 'COGS NZD'
FROM OINV T0
WHERE T0.[DocCur] = 'EUR' and (T0.[DocDate] between [%0] and [%1])
UNION ALL
SELECT T3.[DocNum]
, T3.[DocDate]
, T3.[CardCode]
, T3.[CardName]
, (T3.[DocTotalFC]- T3.[DiscSumFC]- T3.[VatSumFC]) * -1 as 'Total Sales ex GST'
, T3.[VatSumFC] * -1 as 'Total Tax'
, T3.[DocTotalFC] * -1
, T3.[GrosProfFC] * -1
, (T3.[DocTotalFC]- T3.[DiscSumFC]- T3.[VatSumFC]-T3.[GrosProfFC]) as 'COGS NZD'
FROM ORIN T3
WHERE T3.[DocCur] = 'EUR'
and (T3.[DocDate] between [%0] and [%1])
Regards,
Johan