Adding code to old asp page

Hello,

I have some old .asp pages where the user has to make a payment through the website. And, the payment is going through another external site, after approval the variables are returned to my site as session variables. I need to add some more values to the payment site but since I am now on asp.net using Visual Studio 3.5, I am encountering some issues with using .asp extensions. I've changed all of them to .aspx but still I'm not sure what I need to add to make the website work. This old site was developed by someone else.

Thanks ...