I agree with you. I'm in a bit of a fix on this one... the result of trying to be too helpful... :-)
I do not have any experience with zencart. I set up the site as a favor for my sister. Although my first choice was to get her to have the credit card portion running and tested BEFORE I did any custom work she was ademant about "getting on line" so I set it all up, loaded her products, etc. then, rather than give me the info her business partner went in and set up authorize.net. That is who's they won't allow use another URL. Does not make a lot of sense to me either.
Right now if someone orders something it will charge the credit card no problem, get the authorization, etc. It just won't drop the order from inventory or create the order inthe system. If they have an order that someone enters to pay by check it does everything find.
They supposedly called authoize.net who sad it's not their program but an issue with zencart.
I think I found a way I can install zencart into a seperate folder on the server with the URL usingt he latest version if I had to. The only problem would be how to get the data from the mySQL database into the one I created... assuming they can get authorize.net to run properly on the test.
What experience I've had with zencart is that while for me it's a massive pain to change the templates with the right headers, etc. for a store (mostly do to limited documentation), the rest of it (with the modules are pretty simple.
I'll try again with the authorize.net but if you have any thoughts on how to migrate the mysql database to a fresh install let me know.