Hello kind folks,

I am new to Odoo and trying to define a method for computing tax (GST) for my invoices.

I deal in refurbished gadgets and the tax computation is different from regular Indian GST system. In my business,

Tax (GST) = (Sale Price - Purchase Price) * Tax Rate

or Tax (GST) = Margin * Tax Rate

We only charge/pay GST on the gross margin.

I am trying to define this taxation system through the python code option available in the Invoicing app of Odoo. I tried with 

Python Code: result = (price_unit - cost_unit) * 0.09

Applicable Code: result = true

But it seems cost_unit is not recognized by the system. Is there a simple workaround for this? Perhaps by getting the correct reference term for purchase price of the unit?