HTTP 500 at end of checkout ?

[DrByte]

I only use the add-on for Google Analytics (which I think is possibly the problem as I just installed the new simple one). I have zencart 1.3.7 installed. How can I trace the code to find out why this is happening?

What happens if you remove the google analytics code to test ?
Did it work without these errors before you installed that code?

One way to find out exactly what's different between your site and a working "base" install is to compare all your code vs the master code. Here's one documented approach to such comparison:
http://www.zen-cart.com/wiki/index.p...Obscure_Issues

Also I use Network Solutions SSL certificate which is not the best as it doesn't recognize when I am in SSL mode and not.

Clarification: It's not your certificate. It's the Networks Solutions webserver configuration ... their hosting services typically have this problem. There is no solution from a coding perspective. Switching hosts has been the only option for truly engaging SSL support and getting the padlock to show, etc.