Sorry but we have a special tax in Tunisia called "Stamp Fees". This tax is only related to the customer for every invoice posted. (0,400 TND for every invoice depending on customer, the Ministry of Defense don't pay it for example)
It has nothing to do with the materials. All you said is true about taxation and you didn't have to explain this basic concept but thanks for that.
I'm only searching for a system issue to avoid the material classification.