converting an oracle procedure to postgres

I have read the docs on converting Oracle to Postgres already. I'm a little
confused though. I have a procedure in Oracle that just does some
calculations and then does an update based on the calculations. The
procedure doesn't return anything. It seems like in Postgres that everything
has to be a function and has to return something. The following procedure is
almost converted to Postgres format but it is not quite correct yet:
CREATE OR REPLACE PROCEDURE p_updateorders (decimal, date)
AS '
DECLARE
orderno ALIAS FOR $1;
orderdate ALIAS FOR $2;

--defining variables
v_subtotal decimal;
v_taxstatus varchar(1);
v_shipping varchar(12);
v_shippingrate decimal;
V_shippingcharge decimal := 0;
v_taxrate decimal := 0;
v_taxamt decimal;
v_totalamt decimal;
BEGIN
--taking the subtotal by calcualting with right price and qty of
products in an order
SELECT SUM( product_price(orderdate,product_no, qty) * qty) INTO
v_subtotal
FROM orderline
WHERE order_no = orderno
GROUP BY order_no;

--finding if tax applicable or not
SELECT tax_status INTO v_taxstatus
FROM orders
WHERE order_no = orderno;

--finding the shipping method
SELECT shipping_method INTO v_shipping
FROM orders
WHERE order_no = orderno;

--get the tax rate
IF upper(v_taxstatus) = ''Y'' THEN
SELECT tax_rate INTO v_taxrate
FROM tax
WHERE state = (SELECT state
FROM customer WHERE customer_no =
(SELECT distinct customer_no
FROM orders
WHERE order_no = orderno));
END IF;

v_taxamt := v_taxrate * v_subtotal;

--get shipping cost
IF upper(v_shipping) = ''2DAY-AIR'' THEN
v_shippingrate := .08;
ELSIF upper(v_shipping) = ''1DAY-AIR'' THEN
v_shippingrate := .1;
ELSIF upper(v_shipping) = ''GROUND'' THEN
v_shippingrate := .05;
ELSE
v_shippingrate := 0;
END IF;

v_shippingcharge := v_shippingrate * v_subtotal;

--calculating the total amount
v_totalamt := v_subtotal + v_taxamt + v_shippingcharge;

--now update the ORDERS table with new values
UPDATE orders
SET subtotal = v_subtotal,
tax_amt = v_taxamt,
shipping_charge = v_shippingcharge,
total_amt = v_totalamt
WHERE order_no = orderno;

END;
' LANGUAGE 'plpgsql';

I know I have to relpace the word PROCEDURE with FUNCTION but then it wants
me to put RETURNS <datatype> but I don't want to return anything. I was
thinking that I could just have it return integer and then after the last
update statement put return 0. I'd rather not have it return a junk value
though. What should I do?
Thanks,
Clint

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