ERPNext v15 - Country customizations

We need to make customizations to ERPNext core functionalities:

Actual (flat rate) Item specific tax, not as percentage. 
Core functionality only allows to add item specific taxes as a rate (% percentage), in our application we need to pass a tax calculated from item specific variables to other doctypes (Quote, sales order, Sales Invoice)

A complete explanation of the problem can be found here:
https://discuss.frappe.io/t/need-help-for-this-special-ad-valorem-tax-on-alcoholic-beverages/119327


Tax Inclusive calculated from Total instead of Net Total (SOLVED !!) 
****UPDATE**** This requirement has been solved using the following method proposed by ERPNext community:
https://discuss.frappe.io/t/tax-witholding-required-for-vat-and-gst/105749
Basically it's just using a tax with negative number and this calculates correctly the amount on invoice and ledgers.



This applies to quote, sales order, Sales Invoice.



 For detailed explanation, calculation examples and full understanding we need to have a virtual meeting for Q/A.
Thank you in advance for your offerings.

José Echeverria
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