Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, , >= or when the subquery is used as an expression

SELECT TOP (100) PERCENT YEAR(h.InvoiceDate)                                         AS Year,
                         MONTH(h.InvoiceDate)                                        AS Month,
                         h.CONumber,
                         CASE MONTH(InvoiceDate)
                           WHEN 1 THEN '1ST'
                           WHEN 2 THEN '1ST'
                           WHEN 3 THEN '1ST'
                           WHEN 4 THEN '2ND'
                           WHEN 5 THEN '2ND'
                           WHEN 6 THEN '2ND'
                           WHEN 7 THEN '3RD'
                           WHEN 8 THEN '3RD'
                           WHEN 9 THEN '3RD'
                           WHEN 10 THEN '4TH'
                           WHEN 11 THEN '4TH'
                           WHEN 12 THEN '4TH'
                         END                                                         AS Quarter,
                         i.ItemNumber,
                         i.ItemDescription,
                         CASE PRODUCT_GROUP
                           WHEN 00 THEN 'LAB'
                           WHEN 02 THEN 'PRO'
                           WHEN 10 THEN 'ANA'
                         END                                                         AS Division,
                         i.FamilySubgroup,
                         i.FamilyItemNumber,
                         c.CustomerName,
                         h.ShipToDeliveryLocationName,
                         CASE
                           WHEN CustomerCurrencyCode = '00000' THEN 'MYR'
                           ELSE CustomerCurrencyCode
                         END                                                         AS Currency,
                         ISNULL ((SELECT xl.InvoiceForeignUnitPrice
                                  FROM   dbo.FS_ARInvoiceHeader AS xh
                                         LEFT OUTER JOIN dbo.FS_ARInvoiceLine AS xl
                                                      ON xh.ARInvoiceHeaderKey = xl.ARInvoiceHeaderKey
                                  WHERE  ( YEAR(xh.InvoiceDate) >= '2012' )
                                         AND ( xh.InvoiceStatus <> 'X' )
                                         AND ( xh.InvoiceStatus <> 'U' )
                                         AND ( xl.LineItemNumber NOT LIKE 'PACK%' )
                                         AND ( xl.LineItemNumber NOT LIKE 'FREIG%' )
                                         AND ( xh.CustomerID NOT LIKE 'ZZDEMO%' )
                                         AND ( i.ItemNumber IS NOT NULL )
                                         AND ( xh.InvoiceType = 'R' )
                                         AND ( xh.CONumber = h.CONumber )
                                         AND ( xl.LineItemNumber = l.LineItemNumber )
                                         AND ( xh.ShipmentNumberString = h.ShipmentNumberString )
                                         AND ( xh.InvoiceDate > h.InvoiceDate )
                                  GROUP  BY xl.InvoiceForeignUnitPrice), 0)          AS CreditUnitPrice,
                         ISNULL ((SELECT AVG(xl.ShipQuantity) AS Expr1
                                  FROM   dbo.FS_ARInvoiceHeader AS xh
                                         LEFT OUTER JOIN dbo.FS_ARInvoiceLine AS xl
                                                      ON xh.ARInvoiceHeaderKey = xl.ARInvoiceHeaderKey
                                  WHERE  ( YEAR(xh.InvoiceDate) >= '2012' )
                                         AND ( h.InvoiceStatus <> 'X' )
                                         AND ( h.InvoiceStatus <> 'U' )
                                         AND ( l.LineItemNumber NOT LIKE 'PACK%' )
                                         AND ( l.LineItemNumber NOT LIKE 'FREIG%' )
                                         AND ( c.CustomerID NOT LIKE 'ZZDEMO%' )
                                         AND ( i.ItemNumber IS NOT NULL )
                                         AND ( xh.InvoiceType = 'R' )
                                         AND ( xh.CONumber = h.CONumber )
                                         AND ( xl.LineItemNumber = l.LineItemNumber )
                                         AND ( xh.ShipmentNumberString = h.ShipmentNumberString )
                                         AND ( xh.InvoiceDate > h.InvoiceDate )), 0) AS ReturnQty,
                         CASE
                           WHEN InvoiceType = 'I' THEN InvoiceForeignUnitPrice
                           ELSE '0'
                         END                                                         AS InvoiceUnitPrice,
                         l.ShipQuantity,
                         i.MakeBuyCode,
                         CASE MakeBuyCode
                           WHEN 'B' THEN 'N'
                           WHEN 'S' THEN 'N'
                           WHEN 'M' THEN ( CASE CustomerClass4
                                             WHEN '' THEN 'Y'
                                             WHEN '2' THEN 'N'
                                             WHEN '1' THEN ( CASE FamilySubgroup
                                                               WHEN '0205' THEN 'N'
                                                               WHEN '0016' THEN 'N'
                                                               WHEN '0017' THEN 'N'
                                                               WHEN '1021' THEN 'N'
                                                               WHEN '1022' THEN 'N'
                                                               ELSE 'Y'
                                                             END )
                                           END )
                         END                                                         AS Lic
FROM   dbo.FS_ARInvoiceHeader AS h
       LEFT OUTER JOIN dbo.FS_ARInvoiceLine AS l
                    ON h.ARInvoiceHeaderKey = l.ARInvoiceHeaderKey
       LEFT OUTER JOIN dbo.FS_Item AS i
                    ON l.ItemKey = i.ItemKey
       LEFT OUTER JOIN dbo.FS_Customer AS c
                    ON h.CustomerID = c.CustomerID
       LEFT OUTER JOIN dbo.zPRODUCT_CODE AS p
                    ON i.FamilySubgroup = p.PRODUCT_CODE
WHERE  ( YEAR(h.InvoiceDate) >= '2012' )
       AND ( h.InvoiceStatus <> 'X' )
       AND ( h.InvoiceStatus <> 'U' )
       AND ( l.LineItemNumber NOT LIKE 'PACK%' )
       AND ( l.LineItemNumber NOT LIKE 'FREIG%' )
       AND ( c.CustomerID NOT LIKE 'ZZDEMO%' )
       AND ( i.ItemNumber IS NOT NULL )
       AND ( h.InvoiceType = 'I' )
ORDER  BY Month,
          h.CONumber