I copied the includes/classes/payment.php over to a fresh copy of oscommerce and it does not redirect me to the paypal page anymore.
I found out it is the problem of payment.php.

There is a different between those code from install.txt and the one included in the module.

Also, in the install.txt.
the line
payment_value='cot_gv';
should be
payment_value='cot_gv';
for it to works.



More...