FalconAF, WGT confirmed some time ago that this is another bug on this site, namely that some elements on the payment page (to be precise, the ads) are not delivered in a secure way, which is what prevents the whole page from being validated as secure. However, the portions of the webpage where you enter your credit card info, are secure. You can verify this by visiting the payment page in, for example, Internet Explorer 8. When opening the payment page, IE should ask you: "Do you want to open only the secure elements of this webpage?" If you reply, "Yes", the payment page will load, without the ads.
But, a much easier option is using PayPal, and you can forget about the security bugs on this site.