I go to check out in the store via Credit Card payment, and I get a 404 or "page not found" error.

This occurs to some users on Internet Explorer 5.0 or higher, and it is an error due to the way Internet Explorer handles the default configuration for secure connections. Here is the fix to disable TLS 1.0:

From your browser menu:
Select "Tools"
Select "Internet Options"
Select "Advanced"
Scroll down to "Security"
UNCHECK "use TLS 1.0"

Your browser should now use the proper secure protocol and connect to the payment gateway. If you still receive a "page not found" error, please contact us.

When I go to the checkout section, my cart is cleared and it says I have nothing added to my cart.

The best way to work around this is to enable cookies while on the site. Cookies are very important to interface with the store software. Additionally, be sure to have javascript enabled while in the store as it will enable many added store features you can use to make your shopping even easier.