Tax Ammount with a CGI payment module.

I have an cgi payment module where the external (non miva) site is
calculating the tax on the basket. I want to pass this information
back to miva so it shows correctly on the invoice screen. How can I
do this?

Is there something I can add to g.Action with a parameter to make it happen?

Somehow call BasketList_Update_Tax (how exactly?).

All this should (i think?) happen in the translation script the
external site sends the web browser back to.

-Harry