Looping only Once

Your cursor C1 iterates over all matching OCRD records, but inside the loop, you're rejoining OCRD again with OINV, and filtering again by the same CardCode, so the loop essentially becomes redundant because you're fetching data from OCRD again instead of using cur_row.

You should use only the data from the cursor row  inside the loop, and not re-query OCRD so instead, just join OINV with cur_row.CardCode.

 

 

DECLARE CURSOR C1 FOR 
    SELECT 
        T0."CardCode",
        T0."CardName",
        T0."U_AREA",
        T0."U_ROUTE",
        T0."U_Rider_1",
        T0."U_Rider_2",
        T0."U_Rider_3",
        T0."U_Rider_4"
    FROM OCRD T0
    INNER JOIN OCRG T1 ON T0."GroupCode" = T1."GroupCode";

FOR cur_row AS C1 DO
    SELECT 
        cur_row."CardCode",
        cur_row."CardName",
        cur_row."U_AREA",
        cur_row."U_ROUTE",
        T2."DocDate",
        T2."DocNum",
        (SELECT SUM("LineTotal") FROM INV1 WHERE "DocEntry" = T2."DocEntry") AS "Inv Total",
        '' AS "Rider",
        '0' AS "Exp Total"
    FROM OINV T2
    WHERE T2."CardCode" = cur_row."CardCode"
      AND T2."DocDate" >= (:FromDate)
      AND T2."DocDate" <= (:ToDate");
END FOR;