Modify PayPal return URL

Hello,

After a customer completes a payment with PayPal they are redirected back to the invoice. I would like to change this to show the cart.php?a=complete page instead. How can I do that?

When just entering cart.php?a=complete into a browser it redirects to the first step in the order process, so I am not sure if I can just input the url into the return_url tag in the PayPal form? Or is there some setting in the WHMCS admin where you can set this?

Any advise?