i have exactly the same problem. No matter what "order_success" page i put in the parameters in the contribution or/and in the postfinance interface, the landing page after the payment is either a login page or a cart page which is still full of the goods purchased. Moreover, the contribution does not use the process of sending the mail to the customer with the order from oscommerce.
I would be very valuable it somebody could help me in this matter, i guess there is a very small settings problem but as i read in the forum i am not the only one. Any answers or comments are warmly welcome !
Daniel
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