diff --git a/passerelle_imio_extra_fees/models.py b/passerelle_imio_extra_fees/models.py
index 99b2e31..d8f235e 100644
--- a/passerelle_imio_extra_fees/models.py
+++ b/passerelle_imio_extra_fees/models.py
@@ -38,14 +38,19 @@ class ExtraFees(BaseResource):
         data = json.loads(request.body)
 
         nb_documents = 0
+        # Balaie chaque elem du panier.
         for basket_item in data['data']:
+            nb_letter = 0
+            postage_fee = Decimal('0.74')
+            max_doc_in_letter = 5
             try:
                 nb_documents += int(basket_item['request_data']['nb_documents'])
+                if Decimal(basket_item['request_data']['country_price']) > postage_fee:
+                    postage_fee = Decimal(basket_item['request_data']['country_price'])
             except KeyError:
                 # basket item not associated with any document, no fee
                 pass
-
+        nb_letter = int(nb_documents / max_doc_in_letter) + (((nb_documents % max_doc_in_letter) > 0) and 1 or 0)
         # compute fee
-        postage_fee = 3 + nb_documents * 1
-
+        postage_fee = nb_letter * postage_fee
         return {'data': [{'subject': force_text(_('Postage')), 'amount': str(postage_fee)}]}