Do not INSERT if UPDATE fails

Good evening,

I have a custom SQL function in PostgreSQL 9.5.7 which adds a "log entry"
to the table words_payments and then updates "vip_until" column in another
table:

CREATE OR REPLACE FUNCTION words_buy_vip(
in_sid text,
in_social integer,
in_tid text,
in_item text,
in_price float,
in_ip inet)
RETURNS integer AS
$func$
INSERT INTO words_payments (
sid,
social,
tid,
paid,
price,
ip
) VALUES (
in_sid,
in_social,
in_tid,
CURRENT_TIMESTAMP,
in_price,
in_ip
);

UPDATE words_users u
SET vip_until = CURRENT_TIMESTAMP + interval '1 year'
FROM words_social s
WHERE s.sid = in_sid
AND s.social = in_social
AND u.uid = s.uid
AND (u.vip_until IS NULL OR u.vip_until < CURRENT_TIMESTAMP)
RETURNING u.uid;

$func$ LANGUAGE sql;

However if the user record is not found or the user already has vip_until
>= CURRENT_TIMESTAMP (i.e. the user has already purchased "vip status") I
would like to cancel the INSERT.

Is there please a way to rewrite the above function, without switching from
SQL to PL/pgSQL?

Regards
Alex