It seems that since few days, many OSCommerce users have problems with the Paypal payment gateway. Hopefully it was easy to find a solution on Google : you only need to edit the /includes/modules/payment/paypal.php file and replace the line “$this->form_action_url = ‘https://secure.paypal.com/cgi-bin/webscr’;” with “$this->form_action_url = ‘https://www.paypal.com/cgi-bin/webscr’;” (around line 32).