If using Internet Explorer, you may have a problem that has to do with the security settings that Internet Explorer 7.0 defaults to.  It seems to be blocking the Cookie that Paypal uses to checkout.

There is a way to work around it, as follows:
1.  Click on Tools and then Internet Options.  (If you can't see a menu that says Tools, then press the ALT key and it should appear.)
2.  Click on the Privacy tab.
3.  There should be a slider that says "Settings  Select a setting for the internet zone"
4.  Move the slider down, so it is set to Low.  (It is probably set to Medium by default)
5.  Click OK
6.  Navigate back to the page that has the Buy Now button.  Now when you click on the button, you should get to the order page.

Once you have bought the DVD, you can go through the steps to change your security settings back to Medium.