Authorize.net payment notification refused:invalid

I'm using Authorize.net SIM API, and payments complete and show up as Unsettled Transactions at Authorize.net. However, I receive an admin email:

Authorize.net payment notification refused:invalid response

An Authorize.net notification was refused because the response from the Authorize.net server was invalid

I found one mention of this in search, and it says that it can be fixed by changing the transactions to Automatically Settle.

I have two questions about this:

1) Do you know how to set up the Authorize.net transactions to settle so that this error won't appear,

AND

2) How can I modify Hikashop so that it will accept the payment as completed even though it remains an Unsettled Transaction?

Additional info: FYI, I looked up the code where it validates the response MD5, Login ID, Transaction ID and Amount equals what's expected before kicking out this error, so I figured out that I needed to make sure my MD5 and Login IDs are entered correctly. They are. Also, if I change it to API it completes the payment, but I can't use API because a SSL-secured site is not an option for me at the moment!

So, funny story!

Even though I said in my post that I'd verified all my entries between the two, I wanted to do this right. So I went in a literally sixth time to create and apply a new MD5 on both Hikashop and Authorize, this time entering into Authorize first and then copy/pasting that to Hikashop (instead of the other way around). This was important, because I realized something.

I was using an extra character in Hikashop that Authorize doesn't allow! So it really was shorter in Authorize than Hikashop...

Thank you for your reply, it led me to the right answer. And thanks for Hikashop, it's been grand!