confused newbie (to pg not db's)

I have setup a database that I am the owner of, and I am trying to setup
constraints and it doesn't seem to be working

Example

- Table: public."Invoice_Header"

-- DROP TABLE public."Invoice_Header";

CREATE TABLE public."Invoice_Header"
(
"Inv_ID" int4 NOT NULL DEFAULT nextval('public."Invoice
Header_ID_seq"'::text),
"client_ID" int2 NOT NULL,
payable_recievable varchar(5),
"date_CREATED" date,
"AMOUNT" money,
amount_paid money,
date_paid date
) WITH OIDS;

I am trying to make Inv_ID a primary key as follows

alter table Invoice_Header ADD CONSTRAINT PK_inv PRIMARY KEY (Inv_ID)

which runs without error, but nothing changes

this is from psql with PG 7.3.2

any help appreciated