Authorize.net SIM: An error occurred while trying to report this transaction to merch

Hi,

I am having an issue with Authorize.net SIM checkout via Zen Cart 1.3.8a. We would use the AIM version but we are a very small nonprofit company, SSLs aren't cheap, and we aren't experienced at installing one. Our server is a Windows XP machine currently being tested from behind a campus network (we all share one outside IP address). I have plenty of memory and harddrive space. The internet connection is pretty speedy. We are using WAMP server to manage our hosting.

I am having an error wherein when someone clicks the final "submit" button on the Authorize.net page, it tells them:

"An error occurred while trying to report this transaction to the merchant. An e-mail has been sent to the merchant informing them of the error. The following is the result of the attempt to charge your credit card."

Shortly after, I get an email saying (amongst other things):

"Your script timed out while we were trying to post transaction results to
it. Transaction ID: 0
Transaction Result: This transaction has been approved."

I have read some of the other threads similar to mine, but none of them offer workable solutions.

I am wondering if y'all think this might be due to our server currently being behind a campus router. It isn't a live website yet.

Thanks for your help!

I just wanted to add that I have spoken with Authorize.net and they were none too helpful. Can anyone offer some insight into this problem?

Related discussion, which pointed to the server being too slow to respond, or perhaps unable to respond due to firewall issues: http://www.zen-cart.com/forum/showthread.php?t=82164