NAB Transact (Hosted Payment Page) Payment Module

I am using the NAB Transact (Hosted Payment Page) Payment Module (current version), and have updated Zen Cart to the most current. The problem we are having is that when we give a customer a Free Shipping code, it removes the shipping from the total, but it seems to take it off twice.

The invoice I get from Zen Cart is correct, but the NAB Transact invoice is not. It seems to be a problem between them sending the information across. I don't know how to fix this.

It only happens when the customer uses a credit card via the NAB Transact module. If they pay with Paypal the invoice goes through correctly. Any ideas would be great.