validating check digit for Upc code or inserting 12th digit automatically when entering 11 digits

if exists(select 1 
            from INFORMATION_SCHEMA.ROUTINES 
            where ROUTINE_NAME = 'udf_Calculate_UPCA_CheckDigit'
           )
 begin
   drop function dbo.udf_Calculate_UPCA_CheckDigit
 end
go
create function dbo.udf_Calculate_UPCA_CheckDigit
( @PosCode varchar(20)
)
returns varchar(1)
as
begin
/**************************************************************************
2013/01/15 Hakim Ali
Function returns the Check Digit when passed in first 11 characters
of UPC-A.
**************************************************************************/

-- Local variables
declare @checkdigit varchar(1)
declare @sum_of_odds int
declare @sum_of_evens int

-- Initial settings
set @checkdigit = ''
set @PosCode = ltrim(rtrim(isnull(@PosCode,'')))
set @sum_of_odds = 0
set @sum_of_evens = 0

-- Calculate CheckDigit
if (len(@PosCode) = 11 and isnumeric(@PosCode) = 1)
 begin
  set @sum_of_odds = convert(int,substring(@PosCode,1,1))
                   + convert(int,substring(@PosCode,3,1))+ convert(int,substring(@PosCode,5,1))+ convert(int,substring(@PosCode,7,1))+ convert(int,substring(@PosCode,9,1))+ convert(int,substring(@PosCode,11,1))
  set @sum_of_evens = convert(int,substring(@PosCode,2,1))+ convert(int,substring(@PosCode,4,1))+ convert(int,substring(@PosCode,6,1))+ convert(int,substring(@PosCode,8,1))+ convert(int,substring(@PosCode,10,1))

  if ((((@sum_of_odds * 3) + (@sum_of_evens)))%10 = 0)
   begin
    set @checkdigit = '0'
   end
  else
   begin
    set @checkdigit = convert(varchar(1),(10 - (((@sum_of_odds * 3) + (@sum_of_evens)))%10))
   end
 end

return @checkdigit

end -- end create function
go

Code courtesy from sqlzen but I must be doing something wrong cause it does not insert the 12 digit.
I have a table Products and Column PosCode(yes I changed all upca_11 to PosCode

Do i need a stored proc??? this is my first function...

Thanks ahead for any help offered and sorry if I sound like rookie,...I'm just learning..