I've been trying to get Authorize.Net to work for a while now. I got a test account set up. That didn't seem to work with the default payment module, so after more reading here I decided to install the Authorize.net SIM (Payment Form) module since we don't want to handle the credit card numbers anyway. After having no success with that I tried a simple test using the information that Authorize.Net provided just to verify that our test account was working. The simple test script worked, but I still have not been able to get the Authorize.Net SIM module in osCommerce to work. (I'm getting error 14 which says "The referrer, relay response or receipt link URL is invalid." -- As I do more research I'm discovering that to be a rather vague error regarding more than what it states.)
Anyway, I'm going to try out another payment module. Hopefully someone else will provide some feedback about this also.