Torben,
the problem that you see is this:
7 message: ERROR: Unable to identify an operator '+' for types 'double precision' and 'numeric'
You will have to retype this query using an explicit cast
Are you sure you are using PG 7.2 and not a more resent version? Version 7.3 shored up type checking and that is what you encounter here. To PG 4.50 is a double precision data type while shipping_charged is defined as numeric (or vise versa). PG tells you it doesn't know how to add a double precision datatype to a numeric datatype. You'll have to cast one into the other.
/Bart