Authorize.net SIM invoice_num issue

I have installed the Authorize.net SIM paymen module. I can't get it to work. I keep getting this error:

Response Code: 3.
Response Text: Invoice Number is required.Any thoughts? 

Any thought?

Off the top of my head, I'm not sure why it's doing that ... but my first reaction is to suggest using the AIM module instead, since it's more secure and gives the customer a more pleasant and intuitive shopping experience.

I found the problem.
In my Authorize.net admin section there is a form field set up. The invoice field was set to required. I unchecked it and it works fine. I thought this was for the AIM only but I guess the SIM uses it too. HOODATHUNKIT.

I'm going to look into the AIM next. Thanks!

Okay, the original error message you mentioned makes sense then ... if you're using SIM, then the settings in your Authnet account profile will determine how transactions happen. The form field settings there do not affect the AIM module, since AIM handles the entire transaction, but SIM forces authnet to manage form field validation. AIM is certainly better in my opinion ... but you need to protect your site with an SSL certificate (which you really should do anyway, so it's not really a big deal to use AIM then).