Problem with a second subtotal

Problem with a second subtotal

This is an invoice example (I've put in red the part where I have a problem to complete the setup):

Service 1                                   E 50.00

Service 2                                   E 50.00

SUBTOTAL                               E 100.00

Rivalsa Inps 4%                    E    4.00

INVOICE TOTAL                        E 104.00

Ritenuta d'acconto  -20%            E -20.80

TOTAL                                      E  83.20


I have a problem to apply the TAX Ritenuta d'acconto on the Invoice Total and not on the first Subtotal. Is it possible to create a second subtotal on the invoice in which subtract the tax "Ritenuta d'acconto -20%"?
I'm starting to think that your Zoho Invoice couldn't support this feature, but I want to be sure before to abandon this product.


Thanks