Reply To: Redirect to a URL after successful payment

When you are giving access to a page on your site, the best way to handle that is to use a membership type system like the following:

WordPress Membership Plugin – WP eMember

Alternatively, you can use the URL of the page in the “Thank You Page URL” field of that product and the user will get sent to that page after the purchase. However, to apply protection to a WordPress page, you do need a membership type system as mentioned above.