If you want to avoid dynamic SQL, you could use an UDF that will convert comma-delimited string to a table-type variable, and then use IN as usual.

SELECT * FROM customers

WHERE ISNULL(@cust_id_multiple, '') = '' OR customers.cust_id IN (SELECT value FROM dbo.LIST_TO_TABLE(@cust_id_multiple))

Below is a sample UDF that does the trick.

--converts list of values to a temporary table. Used to avoid dynamic SQL statements when IN clause is used as filter expression

--sList: list of numeric values in comma-separated form, to be converted to table

--return value: table-type variable containing one column. The table is filled with numeric values from @sList

CREATE FUNCTION LIST_TO_TABLE (@sList AS VARCHAR(255))

RETURNS  @retTable TABLE(value VARCHAR(50))  AS 

BEGIN

 IF @sList IS NULL RETURN

 --preparing the input list, removing IN keyword and parenthesis

 SET @sList = REPLACE(@sList, 'IN(','')

 SET @sList = REPLACE(@sList, 'IN (','')

 SET @sList = REPLACE(@sList, '(','')

 SET @sList = REPLACE(@sList, ')','')

 SET @sList = LTRIM(RTRIM(@sList))

 --scrolling thru values in @sList and populating temporary table

 DECLARE @index INT, @Delimiter CHAR(1)

 DECLARE @Result VARCHAR(255)

 SET @Delimiter = ','

 WHILE @sList <> ''

 BEGIN

  SET @index = CHARINDEX(@Delimiter, @sList)

  IF @index <> 0

  BEGIN

   SET @Result = LEFT(@sList, @index - 1)

   SET @sList = SUBSTRING(@sList, @index + 1, LEN(@sList))

  END

  ELSE

  BEGIN

   SET @Result = @sList

   SET @sList = ''

  END

  INSERT @retTable SELECT @Result

 END

 RETURN

END