Why do I get a 404 Notfound when checking out of Framemarket shopping cart?

Hi All - I get a 404 Not Found error when I do a test purchase on my website. I'm using FrameMarket/GridMarket themes. (Using authorize.net and they say it's not them, it's the shopping cart software). This is the page that gives me the 404 error:
https://www.highdesertdiva.com/store/store/shopping-cart/shipping/
Help! Thanks!