Checkout is coming up with 404

So here is my issue,
I am running Zen 1.5.0 on a Godaddy server and I am using the Authorize.net (AIM) payment module I have a Standard SSL cert that I used the get the payment mod to work and to get it installed. Now when anyone goes to the store and adds an item to the shopping cart and clicks on the go to checkout button it goes to a 404 screen.
You can test it here (yes this is a live site) http://bonboncandies.com/store/index.php?main_page=product_info&cPath=1&products_id=25
Add the item to the cart and click on the checkout and it pops up. I followed the steps to get the admin area to accept the SSL for the payment gateway to work and it did not say for anything else to do after that. And I clearly missed something so if you can point me to a tutorial or point to where the mess up is it would be greatly appreciated.

Check your HTTPS_SERVER settings in both of your config files by comparing them with the URL that your host has told you to use for SSL.

There appears to be a mismatch somewhere.

Cheers
Rod

Going with what RodG said, it's not only a checkout issue ... it's an SSL configuration issue. When I clicked the Login link I received the message:

Server Error
404 - File or directory not found.
The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

If you can, try to get you own SSL certificate it makes life a lot easier, so the website address is https://bonboncandies.com versus the shared SSL of https://secureserver.net

Thank you everyone for walking me towards the light. The Includes config file was the problem because I failed to have the permissions set to read and write while uploading so it was showing the wrong set up. Nothing like failing to do check the simple things and banging your head on the wall.

You may wish to check your setup slightly, there are some images files that are displayed that are showing as insecure. Also jQuery is not defined properly.