UPDATE

OK, for those of you who are trying to set up eProcessing Network using the AuthorizeNet emulation, the issue may be the new version of authorizenet_aim.php file.

To be fair, I'm thinking the new authorizenet_aim.php file is just fine, but if you try to use it for eProcessing Network, in version 1.3.8a of Zen Cart, it will probably not work.

I installed a fresh version of 1.3.7.1 and patched it. Then I noted the clear differences between the admin set up comparing the two .php files from the two versions of ZC.

I tested my processing in 1.3.7.1 and it worked.

I took the authorizenet_aim.php file from 1.3.7.1 and used that in place of the version that was in 1.3.8a. I changed the URL to hit https://www.eprocessingnetwork.com/cgi-bin/an/order.pl instead of the authorizenet URL and the processing worked.

Important Question
Have I messed anything up using the old file from 1.3.7.1?