diff --git a/__init__.py b/__init__.py
index ea4e609..3507933 100644
--- a/__init__.py
+++ b/__init__.py
@@ -1,5 +1,5 @@
 from trytond.pool import Pool
-from . import product, sale, production
+from . import product, sale, production, invoice
 
 __all__ = ['register']
 
@@ -7,6 +7,7 @@ __all__ = ['register']
 def register():
     Pool.register(
         product.Product,
+        invoice.InvoiceLine,
         sale.Sale,
         sale.Line,
         production.Production,
diff --git a/invoice.py b/invoice.py
new file mode 100644
index 0000000..2a1928e
--- /dev/null
+++ b/invoice.py
@@ -0,0 +1,16 @@
+from trytond.pool import Pool, PoolMeta
+from trytond.model import ModelView, fields
+from trytond.exceptions import UserError
+from trytond.pyson  import Eval
+
+
+class InvoiceLine(metaclass=PoolMeta):
+    __name__ = 'account.invoice.line'
+
+    @classmethod
+    def __setup__(cls):
+        super(InvoiceLine, cls).__setup__()
+
+        cls.product.states['required'] = (Eval('type') == 'line')
+        cls.unit_price.domain = []
+
diff --git a/tryton.cfg b/tryton.cfg
index 8ef765c..7f7e30b 100644
--- a/tryton.cfg
+++ b/tryton.cfg
@@ -6,6 +6,7 @@ depends:
     sale
     sale_supply_production
     production
+    account_invoice
 xml:
     product.xml
     sale.xml
diff --git a/view/sale_form.xml b/view/sale_form.xml
index fe93f27..048d1d7 100644
--- a/view/sale_form.xml
+++ b/view/sale_form.xml
@@ -3,6 +3,7 @@
      this repository contains the full copyright notices and license terms. -->
 <data>
     <xpath expr="//field[@name='reference']" position="after">
+	<newline/>
 	<label name="pizza_number"/>
 	<field name="pizza_number"/>
 	<button name="add_pizza"/>