diff -r 06cf10aa7502 tax.py
--- a/trytond/trytond/modules/account/tax.py	Tue Nov 10 15:00:44 2015 +0100
+++ b/trytond/trytond/modules/account/tax.py	Tue Nov 10 16:38:26 2015 +0100
@@ -3,6 +3,7 @@
 import datetime
 from decimal import Decimal
 from sql.aggregate import Sum
+from itertools import groupby

 from trytond.model import ModelView, ModelSQL, MatchMixin, fields
 from trytond.wizard import Wizard, StateView, StateAction, Button
@@ -366,6 +367,7 @@
         ('fixed', 'Fixed'),
         ('none', 'None'),
         ], 'Type', required=True)
+    update_unit_price = fields.Boolean('Update Unit Price')
     parent = fields.Many2One('account.tax.template', 'Parent')
     childs = fields.One2Many('account.tax.template', 'parent', 'Children')
     invoice_account = fields.Many2One('account.account.template',
@@ -445,6 +447,10 @@
     def default_credit_note_tax_sign():
         return Decimal('1')

+    @staticmethod
+    def default_update_unit_price():
+        return False
+
     def _get_tax_value(self, tax=None):
         '''
         Set values for tax creation.
@@ -453,7 +459,7 @@
         for field in ('name', 'description', 'sequence', 'amount',
                 'rate', 'type', 'invoice_base_sign', 'invoice_tax_sign',
                 'credit_note_base_sign', 'credit_note_tax_sign',
-                'start_date', 'end_date'):
+                'start_date', 'end_date', 'update_unit_price'):
             if not tax or getattr(tax, field) != getattr(self, field):
                 res[field] = getattr(self, field)
         for field in ('group',):
@@ -595,6 +601,9 @@
         ('fixed', 'Fixed'),
         ('none', 'None'),
         ], 'Type', required=True)
+    update_unit_price = fields.Boolean('Update Unit Price',
+        help=('If checked then the unit price for further tax computation will'
+            'be modified by this tax'))
     parent = fields.Many2One('account.tax', 'Parent', ondelete='CASCADE')
     childs = fields.One2Many('account.tax', 'parent', 'Children')
     company = fields.Many2One('company.company', 'Company', required=True,
@@ -745,6 +754,10 @@
         return Decimal('1')

     @staticmethod
+    def default_update_unit_price():
+        return False
+
+    @staticmethod
     def default_company():
         return Transaction().context.get('company')

@@ -770,18 +783,28 @@
                 'tax': self,
                 }

+    def _group_taxes(self):
+        'Key method used to group taxes'
+        return (self.sequence,)
+
     @classmethod
     def _unit_compute(cls, taxes, price_unit, date):
         res = []
-        for tax in taxes:
-            start_date = tax.start_date or datetime.date.min
-            end_date = tax.end_date or datetime.date.max
-            if not (start_date <= date <= end_date):
-                continue
-            if tax.type != 'none':
-                res.append(tax._process_tax(price_unit))
-            if len(tax.childs):
-                res.extend(cls._unit_compute(tax.childs, price_unit, date))
+        for _, group_taxes in groupby(taxes, key=cls._group_taxes):
+            unit_price_variation = 0
+            for tax in group_taxes:
+                start_date = tax.start_date or datetime.date.min
+                end_date = tax.end_date or datetime.date.max
+                if not (start_date <= date <= end_date):
+                    continue
+                if tax.type != 'none':
+                    value = tax._process_tax(price_unit)
+                    res.append(value)
+                    if tax.update_unit_price:
+                        unit_price_variation += value['amount']
+                if len(tax.childs):
+                    res.extend(cls._unit_compute(tax.childs, price_unit, date))
+            price_unit += unit_price_variation
         return res

     @classmethod
diff -r 06cf10aa7502 view/tax_form.xml
--- a/trytond/trytond/modules/account/view/tax_form.xml	Tue Nov 10 15:00:44 2015 +0100
+++ b/trytond/trytond/modules/account/view/tax_form.xml	Tue Nov 10 16:38:26 2015 +0100
@@ -34,6 +34,9 @@
             <label name="company"/>
             <field name="company"/>
             <newline/>
+            <label name="update_unit_price"/>
+            <field name="update_unit_price"/>
+            <newline/>
             <label name="invoice_account"/>
             <field name="invoice_account"/>
             <newline/>
diff -r 06cf10aa7502 view/tax_template_form.xml
--- a/trytond/trytond/modules/account/view/tax_template_form.xml	Tue Nov 10 15:00:44 2015 +0100
+++ b/trytond/trytond/modules/account/view/tax_template_form.xml	Tue Nov 10 16:38:26 2015 +0100
@@ -32,6 +32,9 @@
                 </group>
             </group>
             <newline/>
+            <label name="update_unit_price"/>
+            <field name="update_unit_price"/>
+            <newline/>
             <label name="invoice_account"/>
             <field name="invoice_account"/>
             <newline/>