Originally Posted by CPX
Actually the Thawte certificate is valid. You can see this yourself by visiting the site and then clicking on the Thawte link in the lower left corner. I believe the certificate relies on the IP of the referring website to verify that the request is coming from the website IP that is registered with Thawte.
They do this so that a shady website cannot copy the code from a different website certificate and add it to their website to make it appear legit. Since you are linking from the HP forums (with a different IP referrer) this is most likely the cause of the invalid message.
Furthermore the shopping cart is using an SSL (Secure Socket Layer) https connection which is what Thawte provides as a secure payment gateway. Sorry, eCommerce IT is my business so I just thought I'd chime in.
Another great reason to join HP, such a diverse background of people that you can find out stuff that isn't hookah related.