I found the POST guide for AIM and the XML guide for ARB. http://www.authorize.net/support/CNP/helpfiles/Miscellaneous/Implementation_Guides.htm
Can AIM use XML and POST, and can ARB use XML and POST? Same URLs?
Is there a deprecated method if they are interchangeable?
I found an XML guide for AIM, so AIM supports POST and XML.
I never liked XML, but it works. I will just use XML for both ARB and AIM, rather than two different methods. XML uses one API URL while the POST uses a different API url; so just using one also helps for switching between production/testing easily.
I could use the SDK, but I ran into issues with it in the past.