Am trying to set up a Zen Cart installation I inherited from someone else. The authorize.net account exists and is active. Zen Cart is v1.3.8p3, installed about 2 years ago. The various authorize.net parameters (Login ID, Transaction Key, MD5 Hash) are set properly, as far as I can see, for the SIM method.

When I test the account, it pretty clearly is not talking to authorize.net. I get the Zen Cart page for acquiring the credit card, and without encryption - no SSL connection. Am using the fake credit card numbers that Zen Cart displays for testing use. My "customer" gets a proper transaction-completed email.

The SIM method is supposed to send the customer to an auth.net-served, encrypted page, right?

This must be one of the common problems new users see, but I can't find an existing thread that seems to fit. I've been going over this for some time now, and all the settings appear to be what they should. Can anyone speculate what I'm doing wrong here, or suggest where else to look?