I have made form new form feilds ( big thanks to Absolute ) on the page index.php?main_page=checkout_shipping

The only problem is when you chose the express payment option and log into you acount with paypal you are redirected to

index.php?main_page=checkout_payment

Where can I change this so that they go back to the index.php?main_page=checkout_shipping with the new form feilds ?

Thanks in advance