How to change tax amounts for additional costs on Sage 300 Purchase Orders Steps: Click the Additional Costs tab, and then click the Vendor Taxes button.Edit the tax class if necessary, then click the Calc. Taxes button to see the results of the change.To manually change the total tax amount for the vendor’s additional changes, enter new tax totals in the Tax Amount column.Purchase Orders distributes the updated tax amounts to the additional charges when you move to another tab or post the transaction. Sage 300 Request Support