diff --git a/inema/wpint.py b/inema/wpint.py
index c1b1b77..c5a8302 100644
--- a/inema/wpint.py
+++ b/inema/wpint.py
@@ -9,16 +9,19 @@
 # completely different API with different standards (REST vs. SOAP) with
 # literally nothing in common to the old Internetmarke API
 
-from datetime import datetime, date
-from pytz import timezone
-import requests
-from lxml import etree
 import json
 import logging
+import hashlib
+from datetime import datetime
+
+import requests
+from lxml import etree
+from pytz import timezone
 
 _logger = logging.getLogger(__name__)
 
 class WarenpostInt(object):
+    """Represents the Warenpost Internatoinal ReST interface."""
     def __init__(self, partner_id, key, ekp, pk_email, pk_passwd, key_phase="1", sandbox = False):
         self.sandbox = sandbox
         self.partner_id = 'DP_LT' if sandbox else partner_id
@@ -55,6 +58,7 @@ class WarenpostInt(object):
     # FIXME: merge with inema?
     @staticmethod
     def gen_timestamp():
+        """Generate a timestamp as used in the Warenpsost International API."""
         de_zone = timezone("Europe/Berlin")
         de_time = datetime.now(de_zone)
         return de_time.strftime("%d%m%Y-%H%M%S")
@@ -82,21 +86,21 @@ class WarenpostInt(object):
         auth = requests.auth.HTTPBasicAuth(self.pk_email, self.pk_passwd)
         ret = requests.request('GET', url, headers=self.gen_headers(), auth=auth)
         et = etree.XML(ret.content)
-        e_userToken = et.find(".//{http://oneclickforapp.dpag.de/V3}userToken")
-        e_walletBalance = et.find(".//{http://oneclickforapp.dpag.de/V3}walletBalance")
+        e_user_token = et.find(".//{http://oneclickforapp.dpag.de/V3}userToken")
+        e_wallet_balance = et.find(".//{http://oneclickforapp.dpag.de/V3}walletBalance")
         # update status + return token
-        self.user_token = e_userToken.text
-        self.wallet_balance = e_walletBalance.text
-        _logger.debug("User Token: %s" % (self.user_token))
-        _logger.info("Wallet balance: %s" % (self.wallet_balance))
-        return e_userToken.text
+        self.user_token = e_user_token.text
+        self.wallet_balance = e_wallet_balance.text
+        _logger.debug("User Token: %s", self.user_token)
+        _logger.info("Wallet balance: %s", self.wallet_balance)
+        return e_user_token.text
 
-    def request(self, method, suffix, json=None, headers={}):
+    def request(self, method, suffix, json=None, headers=None):
         """Wrapper for issuing HTTP requests against the API.
            This internally generates all required headers, including Authorization."""
         url = "%s/%s" % (self.url, suffix)
         # FIXME: automatically ensure we have a [current] user_token
-        h = headers.copy()
+        h = headers.copy() if headers else {}
         h.update(self.gen_headers())
         _logger.debug("HTTP Request: %s %s: HDR: %s JSON: %s", method, url, h, json)
         r = requests.request(method, url, json=json, headers=h)
@@ -177,7 +181,8 @@ class WarenpostInt(object):
                 ret['recipientEmail'] = self.email
             return ret
 
-    def build_content_item(self, line_weight_g, line_value, qty, hs_code=None, origin_cc=None, desc=None):
+    def build_content_item(self, line_weight_g, line_value, qty, hs_code=None, origin_cc=None,
+                           desc=None):
         """Build an 'content item' in the language of the WaPoInt API. Represents one
            line on the customs form."""
         ret = {
@@ -193,8 +198,8 @@ class WarenpostInt(object):
             ret['contentPieceOrigin'] = origin_cc
         return ret
 
-    def build_item(self, product, sender, receiver, weight_grams, amount=0, currency='EUR',
-                   contents=[]):
+    def build_item(self, product, sender, recipient, weight_grams, amount=0, currency='EUR',
+                   contents=None):
         """Build an 'item' in the language of the WaPoInt API. Represents one shipment."""
         ret = {
             'product': str(product),
@@ -207,26 +212,26 @@ class WarenpostInt(object):
         # merge in the sender and recipient fields
         ret.update(sender.as_sender())
         ret.update(recipient.as_recipient())
-        if len(contents):
+        if contents:
             ret['contents'] = contents
         return ret
 
-    def build_order(self, items, contactName, orderStatus='FINALIZE'):
+    def build_order(self, items, contact_name, order_status='FINALIZE'):
         """Build an 'order' in the language of the WaPoInt API.  Consists of multiple shipments."""
         ret = {
             'customerEkp': self.ekp,
-            'orderStatus': orderStatus,
+            'orderStatus': order_status,
             'paperwork': {
-                'contactName': contactName,
+                'contactName': contact_name,
                 'awbCopyCount': 1,
                 },
             'items': items
             }
         return ret
 
-    def api_create_order(self, items, contactName, orderStatus='FINALIZE'):
+    def api_create_order(self, items, contact_name, order_status='FINALIZE'):
         """Issue an API request to create an order consisting of items."""
-        order = self.build_order(items, contactName=contactName, orderStatus=orderStatus)
+        order = self.build_order(items, contact_name=contact_name, order_status=order_status)
         _logger.info("Order: %s", order)
         r = self.request('POST', 'orders', json = order)
         # TODO: figure out the AWB and the (item, barcode, voucherId, ...) for the items