diff --git a/kfet/config.py b/kfet/config.py
index deb12504..8e6e0901 100644
--- a/kfet/config.py
+++ b/kfet/config.py
@@ -1,25 +1,40 @@
 # -*- coding: utf-8 -*-
 
+from django.core.exceptions import ValidationError
+
 from djconfig import config
 
 
 class KFetConfig(object):
     """kfet app configuration.
 
-    Enhance dependency with backend used to store config.
+    Enhance isolation with backend used to store config.
     Usable after DjConfig middleware was called.
 
     """
     prefix = 'kfet_'
 
     def __getattr__(self, key):
-        dj_key = '{}{}'.format(self.prefix, key)
-        return getattr(config, dj_key)
+        return getattr(config, self.get_dj_key(key))
 
     def list(self):
         from kfet.forms import KFetConfigForm
         return [(field.label, getattr(config, name), )
                 for name, field in KFetConfigForm.base_fields.items()]
 
+    def get_dj_key(self, key):
+        return '{}{}'.format(self.prefix, key)
+
+    def set_many(self, **kwargs):
+        from kfet.forms import KFetConfigForm
+        new_cfg = KFetConfigForm().initial
+        new_cfg.update({self.get_dj_key(key): value
+                        for key, value in kwargs.items()})
+        cfg_form = KFetConfigForm(new_cfg)
+        if cfg_form.is_valid():
+            cfg_form.save()
+        else:
+            raise ValidationError()
+
 
 kfet_config = KFetConfig()
diff --git a/kfet/views.py b/kfet/views.py
index 96d2a183..34e640b3 100644
--- a/kfet/views.py
+++ b/kfet/views.py
@@ -944,12 +944,12 @@ def kpsul_update_addcost(request):
     account = trigramme and Account.objects.get(trigramme=trigramme) or None
     amount = addcost_form.cleaned_data['amount']
 
-    cfg_form = KFetConfigForm(kfet_addcost_for=account,
-                              kfet_addcost_amount=amount)
-    cfg_form.save()
+    kfet_config.set_many(addcost_for=account and account.pk or None,
+                         addcost_amount=amount)
+
     data = {
         'addcost': {
-            'for': trigramme and account.trigramme or None,
+            'for': account and account.trigramme or None,
             'amount': amount,
         }
     }