While we are integrating Direct Payment Pro(US) setup on our oscommerce site everytime it’s showing an error like “This transaction cannot be processed due to an invalid merchant configuration”. We have checked every configuration in paypal but couldn’t find any problem. Later on a detailed study using paypal sandbox account we could find that in order to test Direct Payment in the Sandbox we have to apply for Website Payments Pro in the Sandbox (we will automatically be approved) then agree to the billing agreement for Website Payments Pro (we will not be charged). And it got worked in the sandbox account. So similar activation is done on the live paypal account also and it starts working.