If you don't want to use another browser other than Internet Explorer 9, here is a way that you can bypass the error.
The button marked below is called the compatibility button in IE9. When you land on the purchase page, just click on the button as shown below. The page might reload. After that if you fill the form, there will be no errors.

In case if the buy now page is not displaying correctly, then click on the 'Show all content' button at the bottom of the explorer after clicking on the 'compatibility button. The page might reload, but it will be displayed correctly then after and no error will come up.





