Discount After Invoice Subtotal in Direct Invoice

I hope that there is an easy answer for this. We do web sales on OpenCart and have offerred a discount (if the customer uses a "special super-secret savings code", that we tell everyone) on the total sales amount (after the discount which may be applied at the line item level).

Providing this "super-secret" discount at the line item level will provide a different total than the OpenCart order total, so I'm at a bit of a loss as to the best way to handle this.  I was hoping that there was an option on the setup menu which would give me another line on the direct sales order screen but I haven't found it.

Has anyone conquered this issue?

Oh, I also tried creating an item of type "discount" and then tried to place it on an invoice with negative cost, but that didn't work because evidently we can't have a negative cost on an invoice. I looked around for an option to change this but haven't found it yet - anyone know of one?

We love FA, by the way and you all have been great!
Jack