I use a payment gateway called NMI. They have a setup for using the Authorize.net AIM and SIM frameworks and processing through them instead of Authorize.net.

They call it Gateway Emulator:

https://secure.networkmerchants.com/gw/merchants/resources/integration/integration_portal.php#carts_authnet I'm wondering if anyone familiar with the Authorize.net code can have  aquick look and see if what I'm thinking is doable? I've played around with making changes as suggested in the Cart section within authorize_request.py but am unable to track down where my various errors are i.e. E_WC_21 & E_WC_19 I think I need to figure out how to modify the js to cooperate? 

Thanks for your time