[ic] discount applied to total-cost instead of subtotal

[interchange-users@interchange.redhat.com (Russ Mann)]

Hello List,

I am using the [discount] tag to effectuate a gift certificate system.
Since a gift certificate is pre-paid, it seems right that taxes, shipping,
and any other charges could be handled by it.  Is there any way, therefore,
to implement a discount based on the [total-cost] instead of the [subtotal]?
I've searched the archives and the docs to no avail.  IC 4.8.3.

Thanks for your help,

-Russ Mann