How to exclude certain line types from Taxable Basis Formula for an Italy withholding tax?

Summary:

Currently, we have an Italy Withholding Tax setup which has a Taxable Basis Formula = STANDARD_TB. When the withholding tax amount is calculated, the withholding tax amount equals the invoice line amounts (including Item, Freight and Miscellaneous lines) multiplied by the tax rate. We would like to exclude Freight and Miscellaneous lines from this formula/calculation. So, the end result would be only the Item line amounts multiplied by the tax rate. What would we need to setup in Oracle to do this?

Content (required):

We looked on the Manage Tax Formulas page but couldn't see a way to exclude freight and miscellaneous from the withholding tax calculation.