when I click on shopping cart CHECKOUT now button I get a 404 file not found page

I am not able to get to a shipping page. The path should be http://www.gmint.org/store/shopping-cart/shipping
That gives a HTTPS: error URL not found on this server.

I just added a couple of products and activated our Authorize.net AIM to test purchase an item and I get this.

I have know idea or time to know how to fix this...
Must be live and working by the weekend for big donor demonstration.