What can be done so that invoice lines with line type freight and miscellaeous lines are not include

Hi All,

Currently the Perception line is entered on invoices as an Item
line.  But Perception is a tax. 

The users tested by entering the Perception line as a Freight line and
as a Miscellaneous line, but it seems that the Withholding tax calculation will
still take these two types of lines to do calculations.

What can be done so the Perception lines are not included in the
Withholding calculation for Argentina Payables Invoices?

Regards,

Raj