Use the instance context to get carrier prices

issue10666
review358661002
(grafted from f3e5c54784ec0b571309a12db6361b6854bdc47c)

--HG--
branch : 6.0

sale.py

@@ -166,7 +166,8 @@ class SaleShipmentCost(metaclass=PoolMeta):
                     raise BadRequest()
             else:
                 carrier = None
-            self.carrier = carrier
+            # Use id to get proper context
+            self.carrier = carrier.id if carrier else None
 
 
 class Line(TaxableMixin, metaclass=PoolMeta):