Hi Dustin,
You're using a version of the Cart which was released in April of last year so it's quite possible that this issue has already been resolved in a past release.
According to our Change Log found
HERE, there was a fix for Single Page Checkout with Direct Gateways with would include Authorize.net.
I recommend downloading and installing the latest trial version of the Cart on a test portal to confirm if the issue still exists or not.
Hope this helps!