I just installed the free version and made some tests with the payment systems. I have set up a Paypal account but after a succesfull payment payal is redirecting to http://www.mydomain.com/order_final.php?tx=4U360887BH161332R&st=Completed&amt=0.60&cc=EUR&cm=&sig=EWcSoOvIP82Ekdu7aFML%2bDOvaJjW 33ELtwVArQDvCxVnM9fJCSJaKuNe26fxQtQl98tHkjC%2f2OtTpzylNROuhBDKcNS6aFEuIEmwS5cUmB1eXbBIsx1ykAma253bcbqqDsngWnfZcqY2A8yT7YVRScYN0P dtDKQSYq6UVBfhJTE%3d
 
This results in a 404 error. Somebody knows what's goung wrong? I have turned on IPN and the return URL with the data transfer option on.

Hello Bas,
 
I think here can be a problems with your settings in PayPal Return URL page, it is should be exact page on your site like http://your_site_url/shop_path/order_final.php so when you typing this address in browser address bar it shows you a page and not 404 error.
 
Hope this helps, Monty