Logic For A Blank Value Help Needed

here's my stored proc and i will tell you what I'm trying to do. I'm passing in a form value. It should handle anything. But it doesn't....
I have tried:
="",
='',
<>"",
<>'',
isNull,
IS NULL,
IS NOT NULL and now the isNumeric() function. Can anyone help me please?!?!

CREATE PROCEDURE dbo.spUpdateProductSpecialPostage

@ProductID as int,
@ScaleID as int


AS

DECLARE @ROWCOUNT as int
select @ROWCOUNT = count(*) from tPostageProduct where productid=@ProductID

if @ROWCOUNT > 0
if isNumeric(@ScaleID) <> 1
delete from tPostageProduct where productid=@ProductID
else
update tPostageProduct set scaleID = @scaleID where productid=@ProductID


else
if isNumeric(@ScaleID) = 1
INSERT INTO tPostageProduct (productId,scaleID) VALUES (@ProductID,@ScaleID)
GO


Cheers.